[gdk-pixbuf] ci: install mingw gtk-doc

[Christoph Reiter <creiter src gnome org>]

commit fafd2b19437afc5e7220ef584fe223245f6d9bd1
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Fri Feb 1 11:02:22 2019 +0100

    ci: install mingw gtk-doc
    
    This should pull in all the xslt/docbook deps we might need

 .gitlab/ci/test-msys2.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/.gitlab/ci/test-msys2.sh b/.gitlab/ci/test-msys2.sh
index 008b7c437..56258b3de 100644
--- a/.gitlab/ci/test-msys2.sh
+++ b/.gitlab/ci/test-msys2.sh
@@ -19,7 +19,8 @@ pacman --noconfirm -S --needed \
     mingw-w64-$MSYS2_ARCH-libpng \
     mingw-w64-$MSYS2_ARCH-libtiff \
     mingw-w64-$MSYS2_ARCH-meson \
-    mingw-w64-$MSYS2_ARCH-toolchain
+    mingw-w64-$MSYS2_ARCH-toolchain \
+    mingw-w64-$MSYS2_ARCH-gtk-doc
 
 meson --buildtype debug \
     -Dx11=false \