tools/tests: allow depriv-fd-checker to build with really old Linux headers

Assuming it was intentional for this test utility, other than most other
ones, to always be built, I think it would be nice if it didn't fail to
build on really old distros just because of the lack of a TUNGETIFF
definition.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/tests/depriv/depriv-fd-checker.c

@@ -330,6 +330,9 @@ static void check_appendonly(void) {
 #include <sys/socket.h>
 #include <linux/if.h>
 #include <linux/if_tun.h>
+#ifndef TUNGETIFF
+#define TUNGETIFF _IOR('T', 210, unsigned int)
+#endif
 
 /* linux tun */