debian: add libx11-xcb-dev as a -dev dependency (for pkgconfig users)

Some users looking for cairo via pkgconfig will notice that libx11-xcb
isn't installed even though cairo's pkgconfig files reference it. Therefore
it should be a dependency, since we're compiling the x11-xcb backend in.

Signed-off-by: Matt Sealey <matt@genesi-usa.com>
bump changelog to encompass font rendering fix & -dev fix

Signed-off-by: Matt Sealey <matt@genesi-usa.com>