BUILD: Fix some clang's warnings == error

CMakeLists.txt

inc/lektor/common.h

inc/lektor/database.h

inc/lektor/net.h

inc/lektor/queue.h

src/database/cache.c

src/database/find.c

src/database/playlist.c

src/database/queue.c

src/database/update.c

src/main/server.c

src/mkv/utils.c

src/net/listen.c

src/net/message.c