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

arch/powerpc/kernel/eeh.c

@@ -1568,6 +1568,7 @@ int eeh_pe_inject_err(struct eeh_pe *pe, int type, int func,
 }
 EXPORT_SYMBOL_GPL(eeh_pe_inject_err);
 
+#ifdef CONFIG_PROC_FS
 static int proc_eeh_show(struct seq_file *m, void *v)
 {
 	if (!eeh_enabled()) {
@@ -1594,6 +1595,7 @@ static int proc_eeh_show(struct seq_file *m, void *v)
 
 	return 0;
 }
+#endif /* CONFIG_PROC_FS */
 
 #ifdef CONFIG_DEBUG_FS