drm/vmwgfx: add extern C guard for the UAPI header

Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Brian Paul <brianp@vmware.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Sinclair Yeh <syeh@vmware.com>