packages/exim: enable parallel make

packages/exim: enable parallel make

Parallel make used to be broken in exim, as reported in its docs. Now that
line has disappeared from the docs, and parallel make is actually working.

Tested with 'make exim-dirclean ; time make BR2_JLEVEL=999 exim': builds
still succeed and the build time decreases from 34 to 11 seconds on my
host.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>