[gdk-pixbuf] meson: Fix no-X11 documentation build

[Emmanuele Bassi <ebassi src gnome org>]

commit 4142b7ffe72a597f7f321dfec0d25b15e4582f1a
Author: Quentin Glidic <sardemff7+git sardemff7 net>
Date:   Sun Apr 1 12:44:51 2018 +0200

    meson: Fix no-X11 documentation build
    
    https://bugzilla.gnome.org/show_bug.cgi?id=794872
    
    Signed-off-by: Quentin Glidic <sardemff7+git sardemff7 net>

 meson.build |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/meson.build b/meson.build
index 13a1e97..e9c7f2d 100644
--- a/meson.build
+++ b/meson.build
@@ -388,6 +388,7 @@ gnome = import('gnome')
 
 subdir('gdk-pixbuf')
 
+gdkpixbuf_xlib_inc = []
 if get_option('x11')
   x11_dep = cc.find_library('X11', required: false)
     if x11_dep.found()