intel: Get intel_decode.c minimally building.

My plan is to use this drm_intel_dump_batchbuffer() interface for the
current GPU tools, and the current Mesa batch dumping usage, while
eventually building more interesting interfaces for other uses.

Warnings are currently suppressed by using a helper lib with CFLAGS
set manually, because the code is totally not ready for libdrm's warnings
setup.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Eugeni Dodonov <eugeni@dodonov.net>