[gnome-build-meta] Restore ref for WebKit

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 0799dd48e13ec1c5388d1ba19a30dc31172ad96a
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Sat Mar 3 20:06:01 2018 -0600

    Restore ref for WebKit
    
    This is supposed to be committed for tarballs; I removed it by mistake.
    
    See also: https://gitlab.com/BuildStream/buildstream/issues/248

 elements/core-deps/WebKit.bst | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/elements/core-deps/WebKit.bst b/elements/core-deps/WebKit.bst
index e8434aa..17d7d45 100644
--- a/elements/core-deps/WebKit.bst
+++ b/elements/core-deps/WebKit.bst
@@ -4,6 +4,7 @@ sources:
 # Reminder: remove -DUSE_UPOWER=OFF when upgrading to 2.19.92
 # https://trac.webkit.org/changeset/228888/webkit
   url: webkitgtk_org:webkitgtk-2.19.91.tar.xz
+  ref: cc1ccc8522bc44e05bf2a7f25b486758298e9ebfbad09fd8a0b5909e45c35c93
 depends:
 - core-deps/at-spi2-core.bst
 - core-deps/brotli.bst