fixup! ABI coder v1 deprecation warning

Author: matheusaaguiar

libsolidity/analysis/SyntaxChecker.cpp

@@ -150,7 +150,10 @@ bool SyntaxChecker::visit(PragmaDirective const& _pragma)
 		else
 			m_sourceUnit->annotation().useABICoderV2 = (_pragma.literals()[1] == "v2");
 
-		if (!*m_sourceUnit->annotation().useABICoderV2)
+		if (
+			m_sourceUnit->annotation().useABICoderV2.set() &&
+			!*m_sourceUnit->annotation().useABICoderV2
+		)
 			m_errorReporter.warning(
 				9511_error,
 				_pragma.location(),