add: strstr

src/string/strstr.c

@@ -0,0 +1,14 @@
+char *strstr(const char *haystack, const char *needle)
+{
+	size_t i = 0;
+	size_t len;
+
+	len = strlen(needle);
+	while (haystack[i] != '\0')
+	{
+		if (strncmp(haystack + i, needle, len) == 0)
+			return haystack + i;
+		i++;
+	}
+	return NULL;
+}