pipe-loader: Include unistd.h in pipe_loader_drm.c for close function.

pipe-loader: Include unistd.h in pipe_loader_drm.c for close function.

This patch fixes a build error on DragonFly.

  CC       libpipe_loader_la-pipe_loader_drm.lo
pipe_loader_drm.c: In function 'pipe_loader_drm_probe':
pipe_loader_drm.c:207:10: error: implicit declaration of function 'close' [-Werror=implicit-function-declaration]

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>