ffmpeg: make X11 dependencies Linux-only

[Akczht]

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ffmpeg: make X11 dependencies Linux-only

libx11 and libxcb are only required for X11 functionality, which is
Linux-specific. macOS builds do not use X11, so move these dependencies
under on_linux to avoid unnecessary requirements on macOS.

[Akczht]

@botantony, sorry for pinging you but I don't know how to proceed now, should this commit be merged?

[Akczht]

the two check failed because they were stuck for a very long time, c=and then they got cancelled

[Akczht]

all except 1 check failed, that too because it's time ran out or it got stuck idk how, but last time I can remember it passed but 2 other failed, but this time those two passed and this one got stuck and then got cancelled

[Akczht]

all checks seem to have passed now