debian/control: Build-Depend on automake >= 1:1.11

We use things only available in automake 1.11+, but dh-autoreconf allows
any automake, so if, say automake1.9 is installed, dh-exec fails to
build. Add an explicit build-dependency on the correct automake version.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>