build: pass -DUSE_ROBIN_HOOD_HASHING to compilers

The file vk_layer_data.h protects the include of robin_hood.h:

    #ifdef USE_ROBIN_HOOD_HASHING
    #include "robin_hood.h"
    #else
    #include <unordered_set>
    #endif

But the -DUSE_ROBIN_HOOD_HASHING=ON|OFF passed to CMake does nothing
to pass the same definition to the compilers.  This change ensures
that the compilers receive -DUSE_ROBIN_HOOD_HASHING if the CMake
switch is on, or nothing if the switch is off.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 804a943..b3f45be 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -252,6 +252,7 @@
 option(USE_ROBIN_HOOD_HASHING "Use robin-hood-hashing" ON)
 if (USE_ROBIN_HOOD_HASHING)
     find_package(robin_hood REQUIRED CONFIG)
+    add_definitions(-DUSE_ROBIN_HOOD_HASHING)
 endif()
 
 if(BUILD_LAYERS OR BUILD_TESTS)