automake: add build dependency on host-autoconf

automake for the target fails to build if autoconf is not installed on
the host:

checking whether autoconf is installed... no
configure: error: Autoconf 2.58 or better is required.
    Please make sure it is installed and in your PATH.
make: *** [/home/test/brbuilttest/build/automake-1.10/.stamp_configured] Error 1

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>