sh: Fix in_nmi symbol build error.

If CONFIG_KGDB_NMI is disabled, we're left with a stray in_nmi
reference that can't be resolved. Move the symbol under the ifdef,
too.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/kernel/cpu/sh3/entry.S

@@ -320,7 +320,9 @@ skip_restore:
 
 	.align	2
 5:	.long	0x00001000	! DSP
+#ifdef CONFIG_KGDB_NMI
 6:	.long	in_nmi
+#endif
 7:	.long	0x30000000
 
 ! common exception handler