build: fix another virdbus issue on mingw

Depending on the set of mingw packages installed, it is possible
that other .c files hit the mingw header pollution from the
virdbus.h file.

In file included from ../../src/rpc/virnetserver.c:39:0:
../../src/util/virdbus.h:41:35: error: expected ';', ',' or ')' before 'struct'
                       const char *interface,
                                   ^

* src/util/virdbus.h (virDBusCallMethod): Match .c file change.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virdbus.h

@@ -38,7 +38,7 @@ int virDBusCallMethod(DBusConnection *conn,
                       DBusMessage **reply,
                       const char *destination,
                       const char *path,
-                      const char *interface,
+                      const char *iface,
                       const char *member,
                       const char *types, ...);
 int virDBusMessageRead(DBusMessage *msg,