package/exim: fix parallel build

exim does build in parallel correctly, but has a concurrency bug in
generating version info files which happens either in the build step or in
the install step.

Add a patch to fix the bug.

Fixes:
  http://autobuild.buildroot.net/results/ebf/ebfccad007e216564889645a07f5487747116331//
  http://autobuild.buildroot.net/results/56a/56a8457efcb32579ad6da99a769b6438dd0db267//
  http://autobuild.buildroot.net/results/6a1/6a1f8a352649baf767b094cb6bbe2a7397fa7fac//
  http://autobuild.buildroot.net/results/5ed/5ed1c42b3d33198f32d1267e5cc2b1fa1211495a//
  http://autobuild.buildroot.net/results/b30/b304569948fd481ce33ecd052a1036153c5d459e//
  http://autobuild.buildroot.net/results/d2c/d2c7abfe08672e53ff890127f787f8d2e84860f4//



Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>