[libchamplain] Don't build vala demo without GTK dependency

[Jiří Techet <jiritechet src gnome org>]

commit 40cdc17b4f851b49f9112a2c467a738bb32d0db3
Author: Jiří Techet <techet gmail com>
Date:   Wed Feb 6 11:58:29 2019 +0100

    Don't build vala demo without GTK dependency

 demos/meson.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/demos/meson.build b/demos/meson.build
index e8bcff1..0f49e5f 100644
--- a/demos/meson.build
+++ b/demos/meson.build
@@ -45,7 +45,7 @@ if build_gtk_widgetry == true
   endforeach
 endif
 
-if generate_vapi == true
+if generate_vapi == true and build_gtk_widgetry == true
   libchamplain_vala_demos = [
     ['launcher-vala', ['launcher-vala.vala', 'markers-vala.vala']],
   ]
@@ -67,4 +67,4 @@ if generate_vapi == true
       ]
     )
   endforeach
-endif
\ No newline at end of file
+endif