Add libsync to dependencies of libgui

This avoids a build break when AOSP change
https://android-review.googlesource.com/c/117076/
merges to internal master.

Test: mmm frameworks/native/libs/gui
Change-Id: I1a1d786e5079d07b5c7e3dedf9010ca783851041
diff --git a/libs/gui/Android.bp b/libs/gui/Android.bp
index bf758ce..9d440cf 100644
--- a/libs/gui/Android.bp
+++ b/libs/gui/Android.bp
@@ -96,6 +96,7 @@
     ],
 
     shared_libs: [
+        "libsync",
         "libbinder",
         "libcutils",
         "libEGL",