multimedia/kdenlive: fix build when multimedia/mlt6 is installed

The build of kdenlive could wrongly pick up the unprefixed headers of
multimedia/mlt6 in ${LOCALBASE}/include due to that path being added
the CFLAGS -- so mlt++/Foo would get you the mlt6 header:

/usr/local/include/mlt++/MltGeometry.h:34:31: [...]

This patch appends ${LOCALBASE}/include at the end, so the prefixed
mlt7 headers should get picked up first.

Reported by: VVD <vvd@unislabs.com>