fix(ft_memcmp): update int to size_t and comply to 42 norm

ft_memcmp.c

@@ -1,16 +1,18 @@
 #include "libft.h"
+#include <stddef.h>
 
 int	ft_memcmp(const void *s1, const void *s2, size_t n)
 {
 	char	*c1;
 	char	*c2;
-	int		i;
+	size_t	i;
 
 	c1 = (char *)s1;
 	c2 = (char *)s2;
 	i = 0;
 	while (i < n && c1[i] == c2[i])
 		i++;
-	if (i == n) return (0);
+	if (i == n)
+		return (0);
 	return (c1[i] - c2[i]);
 }