add solution to hamming

exercises/hamming/hamming.c

 #include "hamming.h"
+#include <string.h>
 
 int compute(const char *lhs, const char *rhs) {
-    // TODO: implement this fucntion
+  int count = 0;
+  if (strlen(lhs) != strlen(rhs)) {
+    return -1;
+  }
+
+  while (*lhs != '\0') {
+    if (*lhs != *rhs) {
+      count++;
+    }
+    lhs++;
+    rhs++;
+  }
+  return count;
 }
+