stream_out: rtsp: use locale.h only if uselocale is available

Like is done in src/config/file.c. It prevents including locale.h in
case it doesn't provide what we need. Instead, vlc_fixup will be used
and uselocale/newlocale will be no-op.
13 jobs for !9 with locale/fixes/1 in 100 minutes and 3 seconds (queued for 58 seconds)
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