layer.conf: Exclude QCOM_COMMON_LICENSE_DIR from hash calculation in LIC_FILES_CHKSUM

Since TOPDIR is appended to the LAYERDIR path contained in this variable,
building from a different directory reduces the sstate-cache hit rate for recipes that use QCOM_COMMON_LICENSE_DIR.

```
TOPDIR1/tmp/sstate-diff/1759133466/qcs6490-rb3gen2-vision-kit/<TRIPLE>/time-genoff $ bitbake-diffsigs ./15.0.do_package.sigdata.28f71c835e5ca367eadc9011ae70403e8cb2bf33b8ae20fa040df5d69dfb6cfe \
TOPDIR2/tmp/sstate-diff/1759133333/qcs6490-rb3gen2-vision-kit/<TRIPLE>/time-genoff/15.0.do_package.sigdata.9eabc9947518db7de3427e8da226b6650ad4b3dad7b6659cbb21245a28460a32
NOTE: Starting bitbake server...
basehash changed from 9990fb107900393658bf819b17528ee4810cefe6d09660e6320eccc4e119dac0 to 327c12e7c95192e2d172d7022c2bdbadd292b5a687b0acc1ab939f19c2f7d216
Variable QCOM_COMMON_LICENSE_DIR value changed from '${@os.path.normpath("TOPDIR1/meta-qcom/meta-qcom-hwe")+'/files/common-licenses/'}' to '${@os.path.normpath("TOPDIR2/meta-qcom/meta-qcom-hwe")+'/files/common-licenses/'}'
```

```bash
List of dependencies for variable LIC_FILES_CHKSUM is ['LICENSE', 'QCOM_COMMON_LICENSE_DIR']
List of dependencies for variable QCOM_COMMON_LICENSE_DIR is []
Variable LIC_FILES_CHKSUM value is file://${QCOM_COMMON_LICENSE_DIR}${LICENSE};md5=58d50a3d36f27f1a1e6089308a49b403
Variable QCOM_COMMON_LICENSE_DIR value is ${@os.path.normpath("TOPDIR1/meta-qcom/meta-qcom-hwe")+'/files/common-licenses/'}
```
The recipe directly depends on `LIC_FILES_CHKSUM`, and `LIC_FILES_CHKSUM` references `QCOM_COMMON_DIR`.

Therefore, excluding `QCOM_COMMON_LICENSE_DIR` from the hash value calculation in `LIC_FILES_CHKSUM` would ideally work.

```bash
TOPDIR2/tmp/sstate-diff/1759299422/qcs6490-rb3gen2-vision-kit/<TRIPLE>/time-genoff $ bitbake-dumpsig ./15.0.do_package.sigdata.763beb6ace3413d57a0a8e15a00d6ec3baf541ebfb050ebae2b627b3f03cecca | grep QCOM_COMMON_LICENSE_DIR
Variable LIC_FILES_CHKSUM value is file://${QCOM_COMMON_LICENSE_DIR}${LICENSE};md5=58d50a3d36f27f1a1e6089308a49b403
TOPDIR2/tmp/sstate-diff/1759299422/qcs6490-rb3gen2-vision-kit/<TRIPLE>/time-genoff $ bitbake-diffsigs ./15.0.do_package.sigdata.763beb6ace3413d57a0a8e15a00d6ec3baf541ebfb050ebae2b627b3f03cecca \
TOPDIR1/tmp/sstate-diff/1759299416/qcs6490-rb3gen2-vision-kit/armv8-2a-atlas-linux/time-genoff/15.0.do_package.sigdata.763beb6ace3413d57a0a8e15a00d6ec3baf541ebfb050ebae2b627b3f03cecca
```

Signed-off-by: mark.yang <[email protected]>

Author: markyang92

conf/layer.conf

@@ -33,6 +33,7 @@ BBFILES_DYNAMIC += " \
 "
 
 QCOM_COMMON_LICENSE_DIR = "${@os.path.normpath("${LAYERDIR}")+'/files/common-licenses/'}"
+LIC_FILES_CHKSUM[vardepsexclude] += "QCOM_COMMON_LICENSE_DIR"
 
 # Set a variable to get to the top of this metalayer
 QCOMHWEBASE = '${@os.path.normpath("${LAYERDIR}")}'