pipe-loader: Add driver build directory for si_driinfo.h include path

pipe-loader: Add driver build directory for si_driinfo.h include path

Fixes out-of-tree build failure:

.../src/gallium/targets/pipe-loader/pipe_radeonsi.c: In function ‘drm_configuration’:
.../src/gallium/targets/pipe-loader/pipe_radeonsi.c:38:33: fatal error: radeonsi/si_driinfo.h: No such file or directory
 #include "radeonsi/si_driinfo.h"
                                 ^
compilation terminated.
Makefile:994: recipe for target 'pipe_radeonsi.lo' failed
make[4]: *** [pipe_radeonsi.lo] Error 1

Trivial.

Fixes: 0f8c5de8690e7c ("radeonsi: prepare for driver-specific driconf
                        options")