selftests/net: rxtimestamp: fix clang issues for target arch PowerPC

The signedness of char is implementation-dependent. Some systems
(including PowerPC and ARM) use unsigned char. Clang 9 threw:
warning: result of comparison of constant -1 with expression of type \
'char' is always true [-Wtautological-constant-out-of-range-compare]
                                  &arg_index)) != -1) {

Tested: make -C tools/testing/selftests TARGETS="net" run_tests

Fixes: 16e7812241 ("selftests/net: Add a test to validate behavior of rx timestamps")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/rxtimestamp.c

@@ -329,8 +329,7 @@ int main(int argc, char **argv)
 	bool all_tests = true;
 	int arg_index = 0;
 	int failures = 0;
-	int s, t;
-	char opt;
+	int s, t, opt;
 
 	while ((opt = getopt_long(argc, argv, "", long_options,
 				  &arg_index)) != -1) {