build: fix virnetlink on glibc 2.11

We were being lazy - virnetlink.c was getting uint32_t as a
side-effect from glibc 2.14's <unistd.h>, but older glibc 2.11
does not provide uint32_t from <unistd.h>.  In fact, POSIX states
that <unistd.h> need only provide intptr_t, not all of <stdint.h>,
so the bug really is ours.  Reported by Jonathan Alescio.

* src/util/virnetlink.h: Include <stdint.h>.

src/util/virnetlink.h

@@ -23,6 +23,8 @@
 # include "config.h"
 # include "internal.h"
 
+# include <stdint.h>
+
 # if defined(__linux__) && defined(HAVE_LIBNL)
 
 #  include <netlink/msg.h>