build: re-add missing CPPFLAGS for libiptc

These got lost on commit v1.4.11-12-g5c8f5b6.

Note: When /usr/include/libiptc/libiptc.h exists, this error is
masked away :-/  (IMO, #include-with-quotes "foo.h" should not
search system dirs...)

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
1 file changed