layer.conf: Correct gcc-cross dependency

The dependency listed in layer.conf is incorrect, gcc-cross DEPENDS on ${TARGET_PREFIX}libc-for-gcc, not virtual/libc. These happen to resolve the same values however they may not always both be built.

The result of this was that gcc-cross gets a different task hash depending on whether virtual/libc was included in the build. Specifically "bitbake m4" and "bitbake virtual/kernel" would result in different task checksums.

The fix is to use the correct dependency name.

[YOCTO #8692]

index 9773632..82d9c41 100644
@@ -41,7 +41,7 @@ SIGGEN_EXCLUDERECIPES_ABISAFE += " \
 "
 
 SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS += " \
-  gcc-cross-${TARGET_ARCH}->virtual/libc \
+  gcc-cross-${TARGET_ARCH}->virtual/${TARGET_PREFIX}libc-for-gcc \
   gcc-cross-${TARGET_ARCH}->linux-libc-headers \
   ppp-dialin->ppp \
   resolvconf->bash \