automake: loader: rework the CPPFLAGS

automake: loader: rework the CPPFLAGS

Rather than duplicating things, just use the generic AM_CPPFLAGS. This
has the fortunate side-effect of adding VISIBILITY_CFLAGS for the dri3
helper. The latter of which was erroneously exposing some internal
symbols.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reported-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Tested-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>