gplugin/gplugin

Fix some include names
bugfix/clang-format-tweaks
2020-02-26, Richard Laager
54917b58582a
Fix some include names

gplugin.h, gplugin-gtk.h, and gplugin-native.h are not in gplugin/ (or
gplugin-gtk/). They are at the top level.
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>