extensions: include strings.h for the definition of ffs()

extensions: include strings.h for the definition of ffs()

This resolves clang compiler warnings:

extensions/libext4_srcs/gen/gensrcs/external/iptables/extensions/libipt_ULOG.c:89:32: error: implicit declaration of function 'ffs' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  printf(" --ulog-nlgroup %d", ffs(loginfo->nl_group));
                               ^
extensions/libext4_srcs/gen/gensrcs/external/iptables/extensions/libipt_ULOG.c:105:9: error: implicit declaration of function 'ffs' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  ffs(loginfo->nl_group));
  ^
extensions/libext_srcs/gen/gensrcs/external/iptables/extensions/libxt_addrtype.c:263:14: error: implicit declaration of function 'ffs' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  int first = ffs(val);
              ^

Test: builds with less warnings
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>