Add FILESYSTEM_MAP[FS_EXTENDED] entry

Now when refreshing the devices, GParted_Core::read_label() calls
GParted_Core::get_fs() with parameter FS_EXTENDED.

Before this change, get_fs() would fail to find file system capabilities
set for FS_EXTENDED 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_EXTENDED and adds this
entry into the FILESYSTEMS vector.  Then get_fs() finds that not
supported capabilities set for FS_EXTENDED 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.

See this earlier commit doing the same thing:
    7870a92b
    Add FILESYSTEM_MAP[FS_UNALLOCATED] entry