selftests/bpf: Fix broken build where char is unsigned

There are architectures where char is not signed. If so, the following
error is triggered:

  | xdp_hw_metadata.c:435:42: error: result of comparison of constant -1 \
  |   with expression of type 'char' is always true \
  |   [-Werror,-Wtautological-constant-out-of-range-compare]
  |   435 |         while ((opt = getopt(argc, argv, "mh")) != -1) {
  |       |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~
  | 1 error generated.

Correct by changing the char to int.

Fixes: bb6a888 ("selftests/bpf: Add options and frags to xdp_hw_metadata")
Signed-off-by: Björn Töpel <[email protected]>
Acked-by: Larysa Zaremba <[email protected]>
Tested-by: Anders Roxell <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

tools/testing/selftests/bpf/xdp_hw_metadata.c

@@ -430,7 +430,7 @@ static void print_usage(void)
 
 static void read_args(int argc, char *argv[])
 {
-	char opt;
+	int opt;
 
 	while ((opt = getopt(argc, argv, "mh")) != -1) {
 		switch (opt) {