TT#6431 Build-Depend on either default-libmysqlclient-dev or libmysql++-dev

Debian stretch will probably ship with MariaDB as default
and libmysql++-dev isn't available in stretch currently,
adjust Build-Dependencies as recommended, see e.g.
http://www.slideshare.net/ottokekalainen/debconf16-bof-on-mariadbmysql-packaging

Change-Id: I176360045701a02df8941d9a5750faaf42899cd8

debian/control

@@ -3,12 +3,12 @@ Section: net
 Priority: optional
 Maintainer: Sipwise Development Team <support@sipwise.com>
 Build-Depends: debhelper (>= 8~),
+               default-libmysqlclient-dev | libmysql++-dev,
                flite-dev,
                libcurl4-openssl-dev | libcurl4-gnutls-dev,
                libev-dev,
                libevent-dev (>= 2.0.0),
                libhiredis-dev,
-               libmysql++-dev,
                libopus-dev,
                libpcre3-dev,
                libspandsp-dev,