cmake: Export the main function from mythfrontend.

This function must be exported so that the Android java loader can
find it and start the application.  This export has no effect on other
platforms.

mythtv/programs/mythfrontend/mythfrontend.cpp

@@ -1846,7 +1846,7 @@ static int revokeRoot (void)
 }
 
 
-int main(int argc, char **argv)
+Q_DECL_EXPORT int main(int argc, char **argv)
 {
     bool bPromptForBackend    = false;
     bool bBypassAutoDiscovery = false;