gplugin/gplugin

Make sure all of the license file headers are pointing to an https link

Testing Done:
Compile and docs.

Reviewed at https://reviews.imfreedom.org/r/127/
usr/include/gplugin-1.0/gplugin-gtk
usr/include/gplugin-1.0/gplugin-gtk.h
debian/tmp/usr/lib/*/libgplugin-gtk.so
debian/tmp/usr/lib/*/pkgconfig/gplugin-gtk.pc
debian/tmp/usr/share/gir-1.0/GPluginGtk-1.0.gir
debian/tmp/usr/share/vala/vapi/gplugin-gtk.deps
debian/tmp/usr/share/vala/vapi/gplugin-gtk.vapi