Fix build when configured --enable-debug

Fix build when configured --enable-debug

Include os.h for ErrorF() to fix implicit-function-declaration warnings when
configured with --enable-debug.

hw/xfree86/parser/DRI.c: In function 'xf86parseDRISection':
hw/xfree86/parser/DRI.c:87:5: error: implicit declaration of function 'ErrorF' [-Werror=implicit-function-declaration]
hw/xfree86/parser/Extensions.c: In function 'xf86parseExtensionsSection':
hw/xfree86/parser/Extensions.c:77:5: error: implicit declaration of function 'ErrorF' [-Werror=implicit-function-declaration]

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Julien Cristau <jcristau@debian.org>