PolicyManager: Use the full path for includes.

This patch drops the -I. option from CPPFLAGS added recently which
forces all the includes to use the full path such as
"update_engine/policy_manager/some_file.h" instead of
"policy_manager/some_file.h". This fixes a bug where scons didn't
detect changes on header files because the '.' directory isn't
listed on the CPPPATH environment variable. Instead of adding it
there, this patch removes it since that's more aligned with the
current codebase style.

BUG=None
TEST=Code builds.

Change-Id: I117f288e5b61c1ac3c953d3ec7a166395068da34
Reviewed-on: https://chromium-review.googlesource.com/185101
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/policy_manager/variable_unittest.cc b/policy_manager/variable_unittest.cc
index f4aa084..8c7bdc5 100644
--- a/policy_manager/variable_unittest.cc
+++ b/policy_manager/variable_unittest.cc
@@ -4,7 +4,7 @@
 
 #include <gtest/gtest.h>
 
-#include "policy_manager/variable.h"
+#include "update_engine/policy_manager/variable.h"
 
 using std::string;