These were built only in the arch indep, and therefore usually x86 only.
Due to that dh:_installman only picked it up there.
Move the make target to the binary section, so that the man pages will
be packaged along the executables.
Update in v2
- keep it in the BUILD_DOC wrapper to be able to skip it with
nodoc build option.
Change-Id: I6aa6f2d78014de7764b6a8804711570b0194f66d
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>