TT#6430 Build-Depend on either default-libmysqlclient-dev or libmysqlclient15-dev

Debian stretch will probably ship with MariaDB as default,
adjust Build-Dependencies as recommended, see e.g.
http://www.slideshare.net/ottokekalainen/debconf16-bof-on-mariadbmysql-packaging

Change-Id: I93e439ba2138cc711e321c2a3717fe04e4b53e31

debian/control

@@ -4,7 +4,7 @@ Priority: extra
 Maintainer: Sipwise Development Team <support@sipwise.com>
 Build-Depends: debhelper (>= 5),
                libglib2.0-dev,
-               libmysqlclient15-dev
+               default-libmysqlclient-dev | libmysqlclient15-dev,
 Standards-Version: 3.9.7
 Homepage: http://sipwise.com/