pidgin/pidgin

Make sure our built headers are marked as source on all of dependency objects. This has been causing issues on the build agents.

Testing Done:
Compiled locally but I've been unable to reproduce the issue locally.

Reviewed at https://reviews.imfreedom.org/r/443/
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)