Hide internal symbols

With gcc, clang and other compilers, symbols are exported per default. The
symbols in discid_private.h, sha1.h and base64.h are for internal use only, so
they are hidden from now on.

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
Merge pull request #13 from sebastinas/hide-private-symbols

Hide internal symbols on Linux and possibly Solaris/BSD.

They are not part of the API.