pidgin/pidgin

Use GPLUGIN_NATIVE_PLUGIN_DECLARE for all libpurple plugins and remove PURPLE_PLUGIN_INIT

Testing Done:
Ran an instance and verified my enabled accounts connected and everything else was queried successfully.

Reviewed at https://reviews.imfreedom.org/r/741/
if ENABLE_DOC
subdir('reference')
endif
if ENABLE_GTK
pidgin_man = configure_file(
input : 'pidgin3.1.in',
output : 'pidgin3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif
if enable_consoleui
finch_man = configure_file(
input : 'finch3.1.in',
output : 'finch3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif