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

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

@@ -1,6 +1,11 @@
 #ifndef __CHARACTER__
 #define __CHARACTER__
 
+#include <stdlib.h>
+#include <string.h>
+
+#include "string.h"
+
 /**
  * @brief Append a character to a string, assuming string points to an array
  * with enough space.
@@ -10,4 +15,13 @@
  */
 void character_append(char* string, char character);
 
+/**
+ * @brief Append a character to a string at a specific index, assuming string points to an array with enough space.
+ *
+ * @param string
+ * @param character
+ * @param index
+ */
+void character_append_at(char* string, const char character, const size_t index);
+
 #endif