add ft_memchr function

ft_memchr.c

@@ -0,0 +1,18 @@
+#include "libft.h"
+
+void	*ft_memchr(const void *s, int c, size_t n)
+{
+	size_t	i;
+	const unsigned char *ptr;
+	
+	i = 0;
+	ptr = (const unsigned char *)s;
+	while (i < n)
+	{
+		if ((unsigned char)c == ptr[i])
+			return ((void*)(ptr+i));
+		i++;
+	}
+
+	return NULL;
+}