bpftool: Remove duplicate string.h header

./tools/bpf/bpftool/sign.c: string.h is included more than once.

Reported-by: Abaci Robot <[email protected]>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=25502
Signed-off-by: Jiapeng Chong <[email protected]>
Acked-by: Quentin Monnet <[email protected]>

Author: wb-zjp846396

tools/bpf/bpftool/sign.c

@@ -11,7 +11,6 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <string.h>
-#include <string.h>
 #include <getopt.h>
 #include <err.h>
 #include <openssl/opensslv.h>