xen/decompress: make helper symbols static

xen/decompress: make helper symbols static

The individual decompression CUs need to only surface their top level
functions to other code. Arrange for everything else to be static, to
make sure no undue uses of that code exist or will appear without
explicitly noticing. (In some cases this also results in code size
reduction, but since this is all init-only code this probably doesn't
matter very much.)

In the LZO case also take the opportunity and convert u8 where lines
get touched anyway.

The downside is that the top level functions will now be non-static
in stubdom builds of libxenguest, but I think that's acceptable. This
does require declaring them first, though, as the compiler warns about
the lack of declarations.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>