Fix -Dwerror=false

Libvirt builds were failing because of:

../src/util/virconf.c: In function ‘virConfGetValueStringList’:
../src/util/virconf.c:950:13: error: Not available before 2.60 [-Werror]
  950 |         G_GNUC_FALLTHROUGH;
      |             ^~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

-Werror was being passed to GCC.

meson -Dwerror=false was being used which is supposed to disable
-Werror, but did not do so.

Apparently because of git being used to manage patches, libvirt thinks
it is being built from git so we need to use:

 -Dwerror=false -Dgit_werror=disabled

to really kill off -Werror.

libvirt.spec

@@ -192,7 +192,7 @@
 %if 0%{?rhel}
     %define enable_werror -Dwerror=true
 %else
-    %define enable_werror -Dwerror=false
+    %define enable_werror -Dwerror=false -Dgit_werror=disabled
 %endif
 
 %define tls_priority "@LIBVIRT,SYSTEM"