spec: Disable -Werror in Fedora RPM builds

Since we switched to using GIT to apply patches in the RPM spec,
we automagically also turned on -Werror, since the .git directory
now exists. We don't want this on in Fedora, since changing
header files often lead to new warnings being issued. Explicitly
turn off -Werror for non-RHEL platforms, instead of relying on
the defaults

libvirt.spec.in

@@ -357,6 +357,8 @@
 # changes in reported warnings
 %if 0%{?rhel}
     %define enable_werror --enable-werror
+else
+    %define enable_werror --disable-werror
 %endif