fix(solutions): memory issues thanks to -fsanitize=address flag with gcc

challenges/caesar-cipher/solutions/c/function/string.h

@@ -1,8 +1,20 @@
 #ifndef __STRING__
 #define __STRING__
 
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define ALPHABET_LENGTH 26
 #define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
+/**
+ * @brief Return the length of a string (excluding '\0').
+ *
+ * @param string
+ */
+size_t string_get_length(const char* string);
+
 /**
  * @brief Shift the alphabet by a given amount.
  *