rpm: set TLS priority to @LIBVIRT,SYSTEM on Fedora >= 25

With newest gnutls available in Fedora 25/rawhide, it is
possible to have TLS priority fallbacks, so we can finally
use --tls-priority=@LIBVIRT,SYSTEM
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

libvirt.spec.in

@@ -206,10 +206,14 @@
     %define enable_werror --disable-werror
 %endif
 
-%if 0%{?fedora} >= 21
-    %define tls_priority "@SYSTEM"
+%if 0%{?fedora} >= 25
+    %define tls_priority "@LIBVIRT,SYSTEM"
 %else
+    %if 0%{?fedora} >= 21
+        %define tls_priority "@SYSTEM"
+    %else
         %define tls_priority "NORMAL"
+    %endif
 %endif