time: Make gmtime_r() visible to Zephyr

The commit 5e4d0c8 made gmtime_r(),
which was previously always visible, only visible to POSIX.

Since gmtime_r() is part of the allowed non-standard libc functions in
the Zephyr Coding Guidelines Rule A.5, make it also visible to Zephyr.

Signed-off-by: Stephanos Ioannidis <[email protected]>

newlib/libc/include/time.h

@@ -228,7 +228,7 @@ int	   getdate_r (const char *, struct tm *);
 
 struct tm *gmtime (const time_t *_timer);
 
-#if __POSIX_VISIBLE
+#if __POSIX_VISIBLE || __ZEPHYR_VISIBLE
 struct tm *gmtime_r (const time_t *__restrict,
                      struct tm *__restrict);
 #endif