[Libguestfs] Use predictable date in POD-generated manpages

[Hilko Bengen]

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