Add FILESYSTEM_MAP[FS_UNALLOCATED] entry

When displaying an unallocated partition
Win_GParted::set_valid_operations() calls GParted_Core::get_fs() with
parameter FS_UNALLOCATED.

Before this change, get_fs() would fail to find file system capabilities
set for FS_UNALLOCATED and construct a not supported capabilities set
and return that.

Afterwards, find_supported_filesystems() creates a not supported
capabilities set from the NULL pointer for FS_UNALLOCATED and adds this
entry into the FILESYSTEMS vector.  Then get_fs() finds that not
supported capabilities set for FS_UNALLOCATED in the FILESYSTEMS vector
and returns that.

This makes no functional difference.  It just seems right as other
unsupported but used file system types have entries in FILESYSTEM_MAP.