libsepol/tests: override CPPFLAGS too

A build toolchain may override CPPFLAGS on the command line of "make".
Doing so currently breaks libsepol/tests compilation, as it requires
"-I../include/ -I$(CHECKPOLICY)" to be provided in gcc's arguments.

This completes commit 15f274073322 ("Makefiles: override *FLAGS and
*LIBS").

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
1 file changed