Fix missing string.h include for cross-platform compatibility

Replace BSD-specific <strings.h> with standard <string.h> which provides
strlen, strcmp, strdup, and strtok on all platforms including Linux.

Author: vinitkumar

c/src/chatbot.c

@@ -1,8 +1,8 @@
 #include <stdlib.h>
 #include <stdio.h>
+#include <string.h>
 #include <ctype.h>
 #include <limits.h>
-#include <strings.h>
 #include "chatbot.h"
 
 // hash table implementation from here