Add missing `sys/types.h` include to `strnstr()` replacement code in
`compat.c` in order to declare `ssize_t` type under musl.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

old url is dead.

Signed-off-by: John Crispin <blogic@openwrt.org>

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@free.fr>

Signed-off-by: Peter Wagner <tripolar@gmx.at>