Include "gtest/gtest.h" using the shorter path

Rely on the -I or -system compiler option to find "gtest/gtest.h". This
makes it easier to build our tests against a copy of gtest outside the
libaom source tree.

Bug: aomedia:356830476, webm:42330726
Change-Id: I9e2c9302d1ff67471523be24da3d2c047447d7e1
diff --git a/test/sum_squares_test.cc b/test/sum_squares_test.cc
index e2c4556..f9174c1 100644
--- a/test/sum_squares_test.cc
+++ b/test/sum_squares_test.cc
@@ -14,7 +14,7 @@
 #include <string>
 #include <tuple>
 
-#include "third_party/googletest/src/googletest/include/gtest/gtest.h"
+#include "gtest/gtest.h"
 
 #include "config/aom_config.h"
 #include "config/aom_dsp_rtcd.h"