Further fixes to Xcode 12.5 support.

Fix macro logic in mvkSupportsBCTextureCompression() for iOS & tvOS platforms.
diff --git a/MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm b/MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm
index ac2ddff..167395d 100644
--- a/MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm
+++ b/MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm
@@ -4175,5 +4175,7 @@
 #endif
 	return true;
 #endif
+#if MVK_IOS_OR_TVOS
 	return false;
+#endif
 }