[gtest] Update BUILD.gn for new .cc file added to gtest build

This is necessary to roll to a googletest rebased on
a83cc11abe4856a60d92ceba2d65af8236cc3500.

INTK-682 #comment [gtest] Update BUILD.gn for new .cc file added to gtest build

Test: CQ
Change-Id: Ic1436aa61bc65695a9ecfff0fa3016955c38f5be
diff --git a/BUILD.gn b/BUILD.gn
index f2ebff9..f15ef60 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -44,6 +44,7 @@
     "googletest/src/gtest-death-test.cc",
     "googletest/src/gtest-filepath.cc",
     "googletest/src/gtest-internal-inl.h",
+    "googletest/src/gtest-matchers.cc",
     "googletest/src/gtest-port.cc",
     "googletest/src/gtest-printers.cc",
     "googletest/src/gtest-test-part.cc",