layers: Fix Windows build warnings

NOEXCEPT is defined multiple times

Change-Id: I9aef970e9588c17071cd71a31873db4090d50b07
diff --git a/layers/descriptor_sets.h b/layers/descriptor_sets.h
index 3505516..4345154 100644
--- a/layers/descriptor_sets.h
+++ b/layers/descriptor_sets.h
@@ -21,6 +21,7 @@
 #define CORE_VALIDATION_DESCRIPTOR_SETS_H_
 
 // Check for noexcept support
+#ifndef NOEXCEPT
 #if defined(__clang__)
 #if __has_feature(cxx_noexcept)
 #define HAS_NOEXCEPT
@@ -40,6 +41,7 @@
 #else
 #define NOEXCEPT
 #endif
+#endif
 
 #include "core_validation_error_enums.h"
 #include "vk_validation_error_messages.h"