mingw: fix build failure

This macro seems to be defined only on linux/unix and it fails during
mingw build. Its value is '16' (taken from net/if.h) so define it if
it's not defined.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/util/virnetdev.c

@@ -52,6 +52,10 @@
 # include <net/if_dl.h>
 #endif
 
+#ifndef IFNAMSIZ
+# define IFNAMSIZ 16
+#endif
+
 #define VIR_FROM_THIS VIR_FROM_NONE
 
 VIR_LOG_INIT("util.netdev");