introspection: Tell the GI scanner to include the C headers

This adds the following line to the generated gir file:

    <c:include name="ibus.h"/>

This is important, for example for Rust bindings generated from the
gtk-rs tooling.

BUG=https://github.com/ibus/ibus/pull/2071

configure.ac

@@ -315,7 +315,7 @@ GOBJECT_INTROSPECTION_CHECK([0.6.8])
 
 IBUS_GIR_SCANNERFLAGS=
 if test x"$found_introspection" = x"yes" ; then
-    IBUS_GIR_SCANNERFLAGS="--warn-all --identifier-prefix=IBus --symbol-prefix=ibus"
+    IBUS_GIR_SCANNERFLAGS="--warn-all --identifier-prefix=IBus --symbol-prefix=ibus --c-include=ibus.h"
     PKG_CHECK_EXISTS([gobject-introspection-1.0 >= 0.9.6],
                      [gir_symbol_prefix=yes],
                      [gir_symbol_prefix=no])