Include GNUInstallDirs module in top-level CMake file

It was accidentally forgotten in the change that introduced support for install rules. This module defines some of the variables that are used by the install rules.

PiperOrigin-RevId: 464915500

CMakeLists.txt

@@ -18,7 +18,7 @@ project(ruy CXX)
 cmake_minimum_required(VERSION 3.13)  # Copied from IREE
 set(CMAKE_CXX_STANDARD 14)
 
-
+include(GNUInstallDirs)
 
 if (PROJECT_NAME STREQUAL CMAKE_PROJECT_NAME)
   set(RUY_IS_TOPLEVEL TRUE)