csserror: Add G_BEGIN_DECLS and G_END_DECLS
The code in gtk/css/gtkcsserror.h shall be surrounded by G_BEGIN_DECLS and G_END_DECLS, or else it's difficult to include it in C++ source files in gtkmm.
The code in gtk/css/gtkcsserror.h shall be surrounded by G_BEGIN_DECLS and G_END_DECLS, or else it's difficult to include it in C++ source files in gtkmm.