drm/i915: make i915_selftest.h self-contained

Fix build breakage:

In file included from <command-line>:
./drivers/gpu/drm/i915/i915_selftest.h:125:1: error: unknown type name ‘bool’
  125 | bool __igt_timeout(unsigned long timeout, const char *fmt, ...);
      | ^~~~

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730181759.26162-1-lucas.demarchi@intel.com