update_engine: Remove dependency on libupdate_engine from the client.

The update_engine_client is a very small DBus client that only
depends on a single utils:: function from the update_engine daemon
codebase. Because of this, it was forced to be linked against many
libraries that it didn't use.

This patch factors out this glib helper function to a new
glib_utils.{cc,h} file and includes only that in the
update_engine_client binary.

BUG=chromium:396440
TEST=FEATURES=test emerge-link update_engine

Change-Id: Icf8d8b3c6ebd22cdb39e6674fb3d9071071ec941
Reviewed-on: https://chromium-review.googlesource.com/209472
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/subprocess.cc b/subprocess.cc
index f2ac17d..fb34267 100644
--- a/subprocess.cc
+++ b/subprocess.cc
@@ -16,7 +16,7 @@
 #include <base/strings/string_util.h>
 #include <base/strings/stringprintf.h>
 
-#include "update_engine/utils.h"
+#include "update_engine/glib_utils.h"
 
 using std::shared_ptr;
 using std::string;