pidgin/pidgin

Fix up the dependencies for our gir files and ensure ours will work as a subproject

Testing Done:
Tested against gplugin 0.34.1 and everything is working well.

Reviewed at https://reviews.imfreedom.org/r/942/
NULL_SOURCES = [
'nullprpl.h',
'nullprpl.c'
]
# nullprpl isn't built by default; when it is built, it's dynamically linked
if DYNAMIC_NULL
null_prpl = shared_library('null', NULL_SOURCES,
dependencies : [libpurple_dep, glib],
install : false, install_dir : PURPLE_PLUGINDIR)
endif