build: fix substitution of RUNSTATEDIR in man pages

build: fix substitution of RUNSTATEDIR in man pages

When RUNSTATEDIR was introduced

  commit d29c917ef470a25149d1f3787ec494d006549f27
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Tue Aug 20 16:05:12 2019 +0100

    src: honour the RUNSTATEDIR variable in all code

The makefile rules for man pages were accidentally not updated for the
new variablle name.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>