[at-spi2-core] configure.ac: make xkb optional, as intended

[Mike Gorse <mgorse src gnome org>]

commit aa3a11a1dece49328e119f8465f0a46cd548b680
Author: Patrick Welche <prlw1 cam ac uk>
Date:   Thu Aug 24 14:49:04 2017 +0100

    configure.ac: make xkb optional, as intended
    
    [] is not sufficient to avoid PKG_CHECK_MODULES' default action of bailing if
    xkbcommon-x11 is not found.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c1de48c..bf3450a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -111,7 +111,7 @@ AS_IF([test "x$enable_x11" = xno], [
       AC_DEFINE(HAVE_XKB, 1, [Define to use XKB])
       X11_CFLAGS="$X11_CFLAGS $XKB_CFLAGS"
       X11_LIBS="$X11_LIBS $XKB_LIBS"
-    ], [])
+    ], [:])
   ])
 
   AC_SUBST(X11_CFLAGS)