configure: fix build error with XA

configure: fix build error with XA

Fixes:

xa_tracker.c: In function 'xa_tracker_create':
 xa_tracker.c:147:5: error: implicit declaration of function 'pipe_loader_drm_probe_fd' [-Werror=implicit-function-declaration]

in some build configurations, as XA now implicitly depends on
gallium_drm_loader.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>