Include <strings.h> for ffs(3).

diff -u -r ../nnn-1.9/nnn.c ./nnn.c
--- ../nnn-1.9/nnn.c	2018-08-10 02:51:49.000000000 +0000
+++ ./nnn.c	2018-09-28 16:57:19.625923395 +0000
@@ -71,6 +71,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <time.h>
 #include <unistd.h>
 #include <readline/history.h>