Correctly #include <gtksourceview/gtksource.h>

Some recent cleanups[1] on gtksourceview have broken our
previously-working use of individual header files.

Include the documented-as-correct header file.

[1]
http://git.gnome.org/browse/gtksourceview/commit/?id=61876d018619c4f8c743a52ad476ce8102b144f6
http://git.gnome.org/browse/gtksourceview/commit/?id=daa7f1b052c1c4e8d5e07c9f3b025e38b9176bb2

https://bugzilla.gnome.org/show_bug.cgi?id=689197