Every time a new <INCLUDE> directive is used inside a gtk-doc
sections.txt file it overrides the current include header until the next
<INCLUDE> directive. This has the unfortunate effect of making every
single section following the print-related ones to generate
documentation that says to include gtkunixprint.h.
In order to avoid re-arranging the gtk3-sections.txt file, we can tell
gtk-doc what's the default header to include for GTK, and override it
using `@Include` directives directly into the gtk-doc stanzas of the
sections that require a different header.
Fixes: #1746
7771869ab3
