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>

src/gallium/auxiliary/pipe-loader/pipe_loader_drm.c

@@ -33,6 +33,7 @@
 #include <fcntl.h>
 #include <stdio.h>
 #include <xf86drm.h>
+#include <unistd.h>
 
 #ifdef HAVE_PIPE_LOADER_XCB