feat: add ft_memset function

libft.h

@@ -1,6 +1,7 @@
 #ifndef LIBFT_A
 #define LIBFT_A
 
+#include <stddef.h>
 # include <stdlib.h>
 
 /*
@@ -47,4 +48,11 @@ int	ft_isprint(int c);
 */
 size_t	ft_strlen(const char *s);
 
+/*
+	The ft_memset() function writes len 
+	bytes of value c (converted to an 
+	unsigned char) to the string b.
+*/
+void	*ft_memset(void *b, int c, size_t len); 
+
 #endif