gtest: Fix include directory install specification

Change-Id: I34bbe88f0d681581967941e7f003d0c38a18cb21
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include-gtest/meson.build b/include-gtest/meson.build
index dfba348..118b208 100644
--- a/include-gtest/meson.build
+++ b/include-gtest/meson.build
@@ -2,4 +2,4 @@
 
 install_headers(
   'stdplus/gtest/tmp.hpp',
-  subdir: 'stdplus')
+  subdir: 'stdplus/gtest')