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: bb6a88885f ("selftests/bpf: Add options and frags to xdp_hw_metadata")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Larysa Zaremba <larysa.zaremba@intel.com>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/r/20231102103537.247336-1-bjorn@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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) {