hwloc: fix race condition in make install

Fixes:
http://autobuild.buildroot.net/results/983/98364cdbffa06f151ab34b301762321b2a251ec6/
http://autobuild.buildroot.net/results/414/41403f8ce4751a27dd1bb9c43f5a97895dea3980/
http://autobuild.buildroot.net/results/d97/d979624843d2d2020cf43770350a8b9a63dcd04f/

Make install contains a race condition in utils/hwloc, as both
install-exec-hook (through intall-exec) and install-data trigger
install-man.

The install-exec-hook target doesn't do anything with the manual pages, so
fix the race condition by dropping the dependency.

Patch sent upstream:
https://www.open-mpi.org/community/lists/hwloc-devel/2015/05/4442.php



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>