Include stdarg.h in compat/vasprintf.h

Ensure we have va_list defined before trying to use it, fixes
compilation problem on Solaris.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4443 d4fa192b-c00b-0410-8231-f00ffab90ce4

include/compat/vasprintf.h

@@ -32,6 +32,8 @@
 #ifndef COMPAT_VASPRINTF_H_INCLUDED
 #define COMPAT_VASPRINTF_H_INCLUDED
 
+#include <stdarg.h>
+
 #ifndef HAVE_VASPRINTF
 int vasprintf(char **strp, const char *fmt, va_list ap);
 #endif