Log Message:
Update mlterm to 3.7.1.

Changes noted in doc/en/ReleaseNote:

ver 3.7.1
* Add "vt_color_mode" to OSC 5380 and 5381.
* Support SKK (--im skk) as an input method plugin.
* Support input method plugins (canna, wnn and skk) on MacOSX/Cocoa.
* Show the number of each conversion candidate on an input method window of wnn.
* Bug fixes:
  Fix compiling error of registobmp. (Enbugged at 3.7.0) (Thanks to Amadeusz)
  Fix https://sourceforge.net/p/mlterm/mailman/message/34992704/
  Fix https://github.com/saitoha/homebrew-sixel/pull/2 (Thanks to @kefir_)
  Fix a bug which dropped almost all glyph complementary fonts.