pidgin/pidgin

PurpleConversationType's values were pascal case when they should be upper snake case

Testing Done:
Ran with the turtles.

Reviewed at https://reviews.imfreedom.org/r/3048/
i18n = import('i18n')
# use the glib preset which is defined at
# https://github.com/mesonbuild/meson/blob/master/mesonbuild/modules/i18n.py#L93
# at the time of this comment.
i18n.gettext(GETTEXT_PACKAGE, preset : 'glib')
devenv.append('PURPLE_LOCALE_DIR', meson.current_build_dir())