layers: Fix an MSVS error and a warning.

Fatal error C1017: invalid integer constant expression
warning C4389: '!=': signed/unsigned mismatch

Change-Id: I13e06f1578e47a0076241e1a344f4f05ae6d50c2
diff --git a/layers/vk_layer_config.h b/layers/vk_layer_config.h
index c4526db..f64c547 100644
--- a/layers/vk_layer_config.h
+++ b/layers/vk_layer_config.h
@@ -44,7 +44,7 @@
     {std::string("VK_DBG_LAYER_ACTION_CALLBACK"), VK_DBG_LAYER_ACTION_CALLBACK},
     {std::string("VK_DBG_LAYER_ACTION_LOG_MSG"), VK_DBG_LAYER_ACTION_LOG_MSG},
     {std::string("VK_DBG_LAYER_ACTION_BREAK"), VK_DBG_LAYER_ACTION_BREAK},
-#if WIN32
+#if defined(WIN32)
     {std::string("VK_DBG_LAYER_ACTION_DEBUG_OUTPUT"), VK_DBG_LAYER_ACTION_DEBUG_OUTPUT},
 #endif
     {std::string("VK_DBG_LAYER_ACTION_DEFAULT"), VK_DBG_LAYER_ACTION_DEFAULT}};