Importing an instance, like JSON, automatically allows the names of all
its functions to be used.

If one of this functions (namely showJSON) is imported separately, GHC 6
is fine with that, but GHC 7 emits a warning (therefore, the compilation of
Ganeti fails because it requires no warnings).

This patch fixes the build for GHC 7 and has no impact on GHC 6.

Note that the same does not apply to the import of readJSON, because that
function is actually used in the same file, so it must be imported separately.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>