Include unistd.h directly by files using it

util/virutil.h bogously included unistd.h. Drop it and replace it by
including it directly where needed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/virnetsockettest.c

@@ -23,6 +23,7 @@
 # include <ifaddrs.h>
 #endif
 #include <netdb.h>
+#include <unistd.h>
 
 #include "testutils.h"
 #include "virutil.h"