Also set QT_STYLE_OVERRIDE

Qt 5.7 has introduced an extra enviornment var for getting Qt/KDE
apps to follow gtk theming so we will export that alongside the
one that we already support.

Thanks to AJSlye for finding reporting and figuring this out.

src/bin/e_main.c

@@ -351,6 +351,7 @@ main(int argc, char **argv)
    /* install qt5ct and overwrite this variable with qt5ct and use that to    */
    /* configure there Qt5 applications.                                       */
    e_util_env_set("QT_QPA_PLATFORMTHEME", "gtk2");
+   e_util_env_set("QT_STYLE_OVERRIDE", "gtk2");
 
    TS("Parse Arguments");
    _e_main_parse_arguments(argc, argv);