Fix dbus to use /bin/sh

[jimmie james]

In ports/devel/dbus/Makefile we find "BUILD_DEPENDS?=       
bash:${PORTSDIR}/shells/bash" which is a new build depend.

According to: http://www.freedesktop.org/Software/dbus  the only
depend is a XML parser "D-BUS' only *required* dependency is an XML
parser (either libxml or expat)."

No notice of change on the freedesktop site, or /usr/ports/UPDATING.

Why are we using GNU shells, when /bin/sh is/should be standard on every system?

Related links:
https://bugs.freedesktop.org/show_bug.cgi?id=1172
https://bugs.freedesktop.org/show_bug.cgi?id=1674