Fixes the following:

    [9/61] Compiling C object 'src/miners/...miner-fs@exe/tracker-miner-files.c.o'.
    ../src/miners/fs/tracker-miner-files.c: In function ‘process_file_cb’:
    ../src/miners/fs/tracker-miner-files.c:2437:11: warning: passing argument 1 of ‘g_free’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]