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/
subdir('libpurple')
subdir('protocols')
if ENABLE_GTK
subdir('pidgin')
else
pidgin_doc = []
endif
if enable_consoleui
subdir('finch')
else
finch_doc = []
endif
alias_target('doc', libpurple_doc, protocols_doc, pidgin_doc, finch_doc)