libvirt: set CFLAGS only for QCOM distros

Restricting CFLAGs modifications using `qcom-distro` override avoid
unnecessary compiler flag modifications in non-QCOM builds.

Signed-off-by: Viswanath Kraleti <viswanath.kraleti@oss.qualcomm.com>

Author: vkraleti

recipes-extended/libvirt/libvirt_git.bbappend

@@ -1,3 +1,3 @@
 # Temporarily disable warnings due deprecated libxml2 APIs until the proper fix
 # gets backported and merged in meta-virtualization
-CFLAGS:append:qcom = " -Wno-deprecated-declarations"
+CFLAGS:append:qcom-distro = " -Wno-deprecated-declarations"