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

challenges/prefix-suffix/solutions/c/function/character.h

@@ -1,6 +1,9 @@
 #ifndef __CHARACTER__
 #define __CHARACTER__
 
+#include <stdlib.h>
+#include <string.h>
+
 /**
  * @brief Append a character to a string, assuming string points to an array
  * with enough space.