contrib: x262: fix cp call

The cp call is broken:

  cp x262/vlc_build/x264.h x262/vlc_build/x262.h
  cp: cannot stat 'x262/vlc_build/x264.h': No such file or directory
  make: *** [../src/x262/rules.mak:37: .x262] Error 1

x264.h isn't generated inside the build directory (x262/vlc_build), but
exists in the x262 source directory. The install rules also install it
from the source directory. Therefore, copy it from and to the source
dir, not the build dir.

Regression from 697a2fcd.
20 jobs for !3146 with x262 in 48 minutes and 8 seconds (queued for 3 seconds)
merge request