Log Message:
libsecret: Avoid to use dollar single quotes in libsecret/Makefile

This should fix the problem reported by Mayuresh via pkgsrc-users@:

 <https://mail-index.NetBSD.org/pkgsrc-users/2018/05/20/msg026746.html>

No PKGREVISION bump since it should only fix the problem on platforms where the
shell does not support dollar single quotes (i.e. `$'...'').

Thanks to <youri> for testing this patch!