[babl] Meson build: generate png image files for docs

[Øyvind Kolås <ok src gnome org>]

commit 0e4853767e04bd5eca8236f29c56f8ef0140491c
Author: John Marshall <jtm home gmail com>
Date:   Sat May 19 11:53:51 2018 +0100

    Meson build: generate png image files for docs

 docs/graphics/meson.build |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/docs/graphics/meson.build b/docs/graphics/meson.build
index 70cd9ab..fad8d10 100644
--- a/docs/graphics/meson.build
+++ b/docs/graphics/meson.build
@@ -43,6 +43,7 @@ if rsvg_convert_bin.found()
         '@INPUT@',
       ],
       install: false,
+      build_by_default: true,
     )
   endforeach
 endif