loader: unconditionally add AM_CPPFLAGS to libloader_la_CPPFLAGS

loader: unconditionally add AM_CPPFLAGS to libloader_la_CPPFLAGS

It seems that due to the conditional autotools is getting confused and
forgetting to add AM_CPPFLAGS when building libloader (when
HAVE_DRICOMMON is not set).

Cc: mesa-stable@lists.freedesktop.org
Fixes: 5a79e0a8e37 "automake: loader: rework the CPPFLAGS"
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>