[Libguestfs] Use predictable date in POD-generated manpages

This is a minor issue that may break co-installability of i386/amd64 Debian packages: If the date in otherwise identical manpages differs across architectures, dpkg will refuse to install the two versions of the package together. See: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=680885 http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=680886 This patch makes for reproducible outcomes. Cheers, -Hilko