Move the gtkunixprint.h include in the gtk-doc stanza

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

docs/reference/gtk/Makefile.am

@@ -276,7 +276,7 @@ GTKDOC_LIBS = \
 
 
 # Extra options to supply to gtkdoc-mkdb
-MKDB_OPTIONS=--output-format=xml --name-space=gtk
+MKDB_OPTIONS=--output-format=xml --name-space=gtk --default-includes=gtk/gtk.h
 
 # Extra SGML files that are included by $(DOC_MAIN_SGML_FILE)
 content_files =					\

docs/reference/gtk/gtk3-sections.txt

@@ -1,6 +1,3 @@
-
-<INCLUDE>gtk/gtk.h</INCLUDE>
-
 <SECTION>
 <FILE>gtkaboutdialog</FILE>
 <TITLE>GtkAboutDialog</TITLE>
@@ -7089,7 +7086,6 @@ GtkPrintOperationPrivate
 
 
 <SECTION>
-<INCLUDE>gtk/gtkunixprint.h</INCLUDE>
 <FILE>gtkprintunixdialog</FILE>
 <TITLE>GtkPrintUnixDialog</TITLE>
 GtkPrintUnixDialog
@@ -7436,7 +7432,6 @@ gtk_print_context_get_type
 
 
 <SECTION>
-<INCLUDE>gtk/gtkunixprint.h</INCLUDE>
 <FILE>gtkprintjob</FILE>
 <TITLE>GtkPrintJob</TITLE>
 GtkPrintJob
@@ -7488,7 +7483,6 @@ gtk_print_job_get_type
 
 
 <SECTION>
-<INCLUDE>gtk/gtkunixprint.h</INCLUDE>
 <FILE>gtkpagesetupunixdialog</FILE>
 <TITLE>GtkPageSetupUnixDialog</TITLE>
 GtkPageSetupUnixDialog

gtk/gtkpagesetupunixdialog.c

@@ -49,6 +49,7 @@
  * SECTION:gtkpagesetupunixdialog
  * @Short_description: A page setup dialog
  * @Title: GtkPageSetupUnixDialog
+ * @Include: gtk/gtkunixprint.h
  *
  * #GtkPageSetupUnixDialog implements a page setup dialog for platforms
  * which don’t provide a native page setup dialog, like Unix. It can

gtk/gtkprintjob.c

@@ -19,6 +19,7 @@
  * SECTION:gtkprintjob
  * @Title: GtkPrintJob
  * @Short_description: Represents a print job
+ * @Include: gtk/gtkunixprint.h
  *
  * A #GtkPrintJob object represents a job that is sent to a
  * printer. You only need to deal directly with print jobs if

gtk/gtkprintunixdialog.c

@@ -64,6 +64,7 @@
  * SECTION:gtkprintunixdialog
  * @Short_description: A print dialog
  * @Title: GtkPrintUnixDialog
+ * @Include: gtk/gtkunixprint.h
  * @See_also: #GtkPageSetupUnixDialog, #GtkPrinter, #GtkPrintJob
  *
  * GtkPrintUnixDialog implements a print dialog for platforms