Handle small final sigma in ibus_keyval_convert_case
Handle small final sigma in ibus_keyval_convert_case

lowercase: GREEK SMALL LETTER FINAL SIGMA (U+03C2)
uppercase: GREEK CAPITAL LETTER SIGMA (U+03A3)

This mapping was correct in UCSConvertCase, but the "legacy" mapping
must also be correct for Caps Lock to work with the final sigma key.

https://gitlab.freedesktop.org/xorg/lib/libx11/commit/7f46a38139

BUG=https://github.com/ibus/ibus/pull/2176

ibus/ibus,ibus/ibus,ibus/ibus,ibus/ibus,fujiwarat/ibus,fujiwarat/ibus,fujiwarat/ibus,fujiwarat/ibus