Support MTLTimestamp on Xcode 11.7.
diff --git a/MoltenVK/MoltenVK/GPUObjects/MVKDevice.h b/MoltenVK/MoltenVK/GPUObjects/MVKDevice.h
index aa5c7d5..c4be1d2 100644
--- a/MoltenVK/MoltenVK/GPUObjects/MVKDevice.h
+++ b/MoltenVK/MoltenVK/GPUObjects/MVKDevice.h
@@ -83,6 +83,9 @@
 const static uint32_t kMVKCachedColorAttachmentCount = 8;
 const static uint32_t kMVKMaxDescriptorSetCount = SPIRV_CROSS_NAMESPACE::kMaxArgumentBuffers;
 
+#if !MVK_XCODE_12
+typedef NSUInteger MTLTimestamp;
+#endif
 
 #pragma mark -
 #pragma mark MVKPhysicalDevice