unix/fatfs_port: Fix month offset in timestamp calculation.

Signed-off-by: Damien George <damien@micropython.org>

ports/unix/fatfs_port.c

@@ -5,7 +5,7 @@ DWORD get_fattime(void) {
     time_t now = time(NULL);
     struct tm *tm = localtime(&now);
     return ((1900 + tm->tm_year - 1980) << 25)
-           | (tm->tm_mon << 21)
+           | ((tm->tm_mon + 1) << 21)
            | (tm->tm_mday << 16)
            | (tm->tm_hour << 11)
            | (tm->tm_min << 5)