gpm: fixes build error when makeinfo is not installed on host

Tested with: make gpm MAKEINFO=no
Should fix http://autobuild.buildroot.net/results/fbf/fbf42eac7cf09b2f3fc93fa2134ddefcb1beea2f/build-end.log


Thanks to Romain Naour for pointing out the problem.

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>