gdb: do not build documentation for bfd and gprof

Our previous patch to disable the documentation in gdb wasn't complete.
There are cases where the documentation under bfd and gprof directories
is being built, causing the subsequent failure due to missing makeinfo.
This patch fixes that problem.

Fixes:

  http://autobuild.buildroot.org/results/244/2442e697d8a300496434fd42fcb1ee3941d13e06/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>