gdbus-codegen: Add support for pragma inclusion guard

The #pragma once is widely supported preprocessor directive that can
be used instead of include guards.

This adds support for using optionally this directive instead of
include guards.

https://bugzilla.gnome.org/show_bug.cgi?id=791015

docs/reference/gio/gdbus-codegen.xml

@@ -35,6 +35,7 @@
     <arg><option>--c-generate-autocleanup</option> none|objects|all</arg>
     <arg><option>--output-directory</option> <replaceable>OUTDIR</replaceable></arg>
     <arg><option>--generate-docbook</option> <replaceable>OUTFILES</replaceable></arg>
+    <arg><option>--pragma-once</option></arg>
     <arg><option>--xml-files</option> <replaceable>FILE</replaceable></arg>
     <group choice="plain" rep="repeat">
       <arg>
@@ -215,6 +216,17 @@
       </listitem>
     </varlistentry>
 
+    <varlistentry>
+      <term><option>--pragma-once</option></term>
+      <listitem>
+        <para>
+          If this option is passed, the
+          <ulink url="https://en.wikipedia.org/wiki/Pragma_once">&#35;pragma once</ulink>
+          preprocessor directive is used instead of include guards.
+        </para>
+      </listitem>
+    </varlistentry>
+
     <varlistentry>
       <term><option>--c-generate-object-manager</option></term>
       <listitem>