src/meta-openembedded:libmemcached: ignore CVE-2023-27478

Per [1] this is fixed by [2].
The commit message says that it is reverting feature added in:

$ git tag --no-contains d7a0084 | grep 1.0.18
1.0.18

This recipe is for the original memcached which is unmaintained now.
Hence the ignore instead of upgrade.

[1] https://nvd.nist.gov/vuln/detail/CVE-2023-27478
[2] awesomized/libmemcached@48dcc61a

Signed-off-by: Peter Marko <peter.markosiemens.com>
Signed-off-by: Khem Raj <raj.khemgmail.com>