tools/nolibc/string: remove open-coded strnlen()

The same header already defines an implementation of strnlen(),
so use it.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/include/nolibc/string.h

@@ -187,12 +187,7 @@ char *strndup(const char *str, size_t maxlen)
 static __attribute__((unused))
 size_t strlcat(char *dst, const char *src, size_t size)
 {
-	size_t len = 0;
-
-	for (; len < size; len++) {
-		if (dst[len] == '\0')
-			break;
-	}
+	size_t len = strnlen(dst, size);
 
 	/*
 	 * We want len < size-1. But as size is unsigned and can wrap