link.c: fixed to include <sys/types.h> for caddr_t

When building against musl libc compiling bails out because of caddr_t not
being declared:
--------------------------------------------------------------------------
link.c: In function 'wireless_sigqual':
link.c:41:24: error: 'caddr_t' undeclared (first use in this function)
--------------------------------------------------------------------------

As caddr_t is declared in <sys/types.h> this header needs to be included.