Make gfxstream visible to rutabaga

We now make the gfxstream backend visible for other targets. This will
allow rutabaga to depend on the gfxstream backend.

Change-Id: Ic87bb6621704bf79bca4e82d697fe9d30e7ff770
diff --git a/host/BUILD.bazel b/host/BUILD.bazel
index 9942825..effb33f 100644
--- a/host/BUILD.bazel
+++ b/host/BUILD.bazel
@@ -166,6 +166,7 @@
         "WIN32_LEAN_AND_MEAN",
     ],
     linkstatic = True,
+    visibility = ["//visibility:public"],
     deps = [
         ":gfxstream_backend_static",
         "//hardware/google/aemu/base:aemu-base",