[host] app: add os_showMessage for Linux

Linux doesn't have a generic way to show a dialog box, so we just use
DEBUG_INFO to print the message out to the console.

host/platform/Linux/src/platform.c

@@ -77,3 +77,8 @@ bool os_blockScreensaver()
 {
   return false;
 }
+
+void os_showMessage(const char * caption, const char * msg)
+{
+  DEBUG_INFO("%s: %s", caption, msg);
+}