Do not build can-loopback when unsupported

The example only works on devices with CAN peripheral

Author: timokroeger

Cargo.toml

@@ -132,6 +132,10 @@ required-features = ["rt"]
 name = "can-echo"
 required-features = ["connectivity"]
 
+[[example]]
+name = "can-loopback"
+required-features = ["stm32f103", "stm32f105", "stm32f107"]
+
 [[example]]
 name = "can-rtfm"
 required-features = ["connectivity", "rt"]