Add missing config.h includes.

Add missing config.h includes.

conf.h and start.h weren't explicitly including config.h which meant that
depending on the ordering of the includes in whatever was including conf.h
or start.h, some pieces of the structs defined in those may be missing.

This led amongst other problems to the lxc_conf struct being wrong by 8 bytes
for functions from commands.c, leading to lxc-stop always failing.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>