Fix compilation of Xorg DDX without XF86VIDMODE since 6e74fdda, by putting
xf86vmode.c back under the XF86VIDMODE automake conditional it was accidentally
taken out of.
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Tested-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>