Export QLatin1::convertFromUnicode()

We need it in QtNetwork.

Change-Id: I6d26592ee79bffc7390ea742892f134e69ad77b1
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qstringconverter_p.h

@@ -52,6 +52,7 @@ struct QLatin1
     static char *convertFromUnicode(char *out, QStringView in, QStringConverter::State *state) noexcept;
 
     // Defined in qstring.cpp
+    Q_CORE_EXPORT
     static char *convertFromUnicode(char *out, QStringView in) noexcept;
 };