Pass MTP_INCLUDE_DIR to include_directories when trying to link against libmtp

cmake/FindMtp.cmake defines MTP_INCLUDE_DIR, yet it is not used while trying
to link against libmtp in mtp/CMakeLists.txt. This makes it fail with
| /wrkdirs/usr/ports/devel/kio-extras/work/kio-extras-15.12.0/mtp/devicecache.h:32:10: fatal error: 'libmtp.h' file not found
| #include <libmtp.h>

This patch passes it to include_directories, thereby fixing the build on FreeBSD.

REVIEW: 126599