Include string.h, instead of strings.h

This fixes warning: implicit declaration of function ... 

Ref: https://qa.debian.org/bls/bytag/W-implicit-declaration.html

xmountains.c

@@ -1,7 +1,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <strings.h>
+#include <string.h>
 #include <signal.h>
 #include <sys/types.h>
 #include <unistd.h>