Fix compilation on 32-bit systems

Compilation on 32-bit bombs out due to uint64_t not being the same size as a pointer:

    cc -g -O2 -std=c99 -Wall -Werror -D_FILE_OFFSET_BITS=64   -c -MD -o notmuchfs.o notmuchfs.c
    notmuchfs.c: In function ‘notmuchfs_opendir’:
    notmuchfs.c:493:13: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]

Of course storing pointers in a bigger integer is perfectly safe so we just have to make these warnings non-fatal.