powerpc/eeh: Fix build failure with CONFIG_PROC_FS=n

The build fails with CONFIG_PROC_FS=n:

  arch/powerpc/kernel/eeh.c:1571:12: error: ‘proc_eeh_show’ defined but not used
   1571 | static int proc_eeh_show(struct seq_file *m, void *v)

Wrap proc_eeh_show() in an ifdef to avoid it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210314093300.131998-1-mpe@ellerman.id.au