pci_ids: no not include loader.h

As per original approach by Rob, each user of the loader lib should include
loader.h and the pci_id_driver_map.h header will be used exclusively by the
loader.

Add back the include guard __IS_LOADER and remove no longer needed include
folder in the scons build.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/loader/SConscript

@@ -6,7 +6,6 @@ if not env['drm']:
 env = env.Clone()
 
 env.Prepend(CPPPATH = [
-    '.',
     '#include'
 ])