gplugin/gplugin

Don't explicitly use shared_library on gplugin, gplugin-gtk3, and gplugin-gtk4

By just using the library function meson can choose to static link these into
another target.

Testing Done:
Did intial testing in the pidgin tree by hacking `gplugin/meson.build` and used `ldd` to verify that gplugin was staticly linked and that plugins still worked.

Reviewed at https://reviews.imfreedom.org/r/1346/
/*
* Copyright (C) 2021 Elliott Sales de Andrade <quantum.analyst@gmail.com>
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, see <https://www.gnu.org/licenses/>.
*/
#ifndef GPLUGIN_GTK_H
#define GPLUGIN_GTK_H
#ifndef __GI_SCANNER__
# ifdef GPLUGIN_GTK_COMPILATION
# error "gplugin-gtk source files should not include gplugin-gtk.h"
# endif /* GPLUGIN_GTK_COMPILATION */
#endif /* __GI_SCANNER__ */
#ifndef GPLUGIN_GTK_GLOBAL_HEADER_INSIDE
# define GPLUGIN_GTK_GLOBAL_HEADER_INSIDE
#endif /* GPLUGIN_GTK_GLOBAL_HEADER_INSIDE */
@GPLUGIN_GTK_H_INCLUDES@
#undef GPLUGIN_GTK_GLOBAL_HEADER_INSIDE
#endif /* GPLUGIN_GTK_H */