pgrep: prevent buffer overflow in opts

Recent commit 9742c74e7c ("pgrep: Enable case-insensitive process matching")
caused the "opts" string to overflow the show 32-character space allocated for
it.

Bump it up to 64 bytes, which should be enough even if more options are added.

Tested: Running ./pgrep stopped crashing and `make check` passed.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

pgrep.c

@@ -677,7 +677,7 @@ static int signal_option(int *argc, char **argv)
 
 static void parse_opts (int argc, char **argv)
 {
-	char opts[32] = "";
+	char opts[64] = "";
 	int opt;
 	int criteria_count = 0;