hp2ps: install shell wrapper

Before this change we installed hp2ps both
to inplace/bin/ and ${prefix}/bin/

In both cases we added $(CrossCompilePrefix) as
a binary prefix. It's incorrect for inplace install
as none of inplace binaries are prefixed.

The change it to track 'hp2ps' as unprefixed binary.
$(CrossCompilePrefix) prefix is only added to the
installed shell wrapper.

Now 'hp2ps' is handled in a similar way to 'hpc' and
'ghc-pkg'.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>