Fix make -j builds

Fix make -j builds

In files generated by mpers scripts, includes directives are taken from
original files where the type definition was done.  This causes to
include defs.h in multiple files.  defs.h includes printers.h header,
which is a generated header.

This patch add an explicit dependency to printers.h for the mpers
scripts.

* Makefile.am (mpers-m%.stamp): Add printers.h to order-only
prerequisites.

Signed-off-by: Gabriel Laskar <gabriel@lse.epita.fr>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>