Export libicuuc headers

libxml2 headers reference libicuuc headers, reexport them.

Test: m -j checkbuild
Change-Id: I93912948a8144af2f9ec1ac66403c1702027473c
diff --git a/Android.bp b/Android.bp
index d3f313b..dcbd464 100644
--- a/Android.bp
+++ b/Android.bp
@@ -95,4 +95,5 @@
         },
     },
     shared_libs: ["libicuuc"],
+    export_shared_lib_headers: ["libicuuc"],
 }