time/ISO8601: don't use glibc extension in strptime.

Per the manual for strptime, %F is equivalent %Y-%m-%d, so use that
directly.

NEWS

@@ -1,4 +1,6 @@
 ver 0.22.7 (not yet released)
+* protocol
+  - don't use glibc extension to parse time stamps
 * decoder
   - ffmpeg: fix build problem with FFmpeg 3.4
 * storage

src/time/ISO8601.cxx

@@ -185,7 +185,7 @@ ParseISO8601(const char *s)
 	struct tm tm{};
 
 	/* parse the date */
-	const char *end = strptime(s, "%F", &tm);
+	const char *end = strptime(s, "%Y-%m-%d", &tm);
 	if (end == nullptr) {
 		/* try without field separators */
 		end = strptime(s, "%Y%m%d", &tm);