[gdk-pixbuf] meson: do not make x11 required

[Ignacio Casal Quinteiro <icq src gnome org>]

commit f38e3e2cededb3ed9d8887072a4e685d3057847e
Author: Ignacio Casal Quinteiro <qignacio amazon com>
Date:   Tue Jan 16 11:30:29 2018 +0100

    meson: do not make x11 required
    
    On Windows we do not have x11

 meson.build |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/meson.build b/meson.build
index d254dde..2f7b5d3 100644
--- a/meson.build
+++ b/meson.build
@@ -389,7 +389,7 @@ gnome = import('gnome')
 subdir('gdk-pixbuf')
 
 if get_option('x11')
-  x11_dep = cc.find_library('X11')
+  x11_dep = cc.find_library('X11', required: false)
     if x11_dep.found()
       subdir('contrib/gdk-pixbuf-xlib')
     endif