UI: Fix warning when compiling with Clang 10

Building with Clang 10 on FreeBSD 13-CURRENT emitted a warning about
logical not (!) applying only to the left hand side of a comparison.
Update the expression to match the style of other flags conditionals.
Verified that object code is the same.

UI/window-basic-main.cpp

@@ -7264,7 +7264,7 @@ void OBSBasic::on_actionCopySource_triggered()
 		copyVisible = obs_sceneitem_visible(item);
 
 		uint32_t output_flags = obs_source_get_output_flags(source);
-		if (!(output_flags & OBS_SOURCE_DO_NOT_DUPLICATE) == 0)
+		if (output_flags & OBS_SOURCE_DO_NOT_DUPLICATE)
 			allowPastingDuplicate = false;
 	}