[apidiff] Don't try to update legacy refs unless legacy is enabled. (#16363)

tools/apidiff/Makefile

@@ -512,6 +512,7 @@ ifdef ENABLE_DOTNET
 update-dotnet: $(DOTNET_REFS)
 endif
 
+ifdef INCLUDE_XAMARIN_LEGACY
 ifdef INCLUDE_IOS
 update-refs: $(IOS_REFS)
 endif
@@ -524,6 +525,7 @@ endif
 ifdef INCLUDE_MAC
 update-refs: $(MAC_REFS)
 endif
+endif
 
 ifdef ENABLE_DOTNET
 update-refs: $(DOTNET_REFS)