conf/nwfilter: Initialize size_t attr to zero

Newer GCC (13.1.1 in my case) wrongly reports "maybe uninitialized"
warning for this variable inside the next condition.  Even though this
accusation is wrong (the condition is guarded by the same condition as
the for cycle initializing it), initialize it during the declaration so
compilation errors don't stop others and maybe also future proof the
code for changes.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

src/conf/nwfilter_conf.c

@@ -2393,7 +2393,7 @@ virNWFilterRuleParse(xmlNodePtr node)
 
     if (nattrNodes > 0) {
         size_t i;
-        size_t attr;
+        size_t attr = 0;
 
         /* First we look up the type of the first valid element. The rest of
          * the parsing then only considers elements with same name. */