add vhangup syscall wrapper

request/patch by william haddonthethird, slightly modifed to add
_GNU_SOURCE feature test macro so that the compiler can verify the
prototype matches.

src/linux/vhangup.c

+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int vhangup(void)
+{
+	return syscall(SYS_vhangup);
+}