powerpc/64s: Include <asm/nmi.h> header file to fix a warning

Make sure to include <asm/nmi.h> to provide the following prototype:
hv_nmi_check_nonrecoverable.

Remove the following warning treated as error (W=1):

  arch/powerpc/kernel/traps.c:393:6: error: no previous prototype for 'hv_nmi_check_nonrecoverable'

Fixes: ccd47702 ("powerpc/64s: Fix HV NMI vs HV interrupt recoverability test")
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -71,6 +71,7 @@
 #include <sysdev/fsl_pci.h>
 #include <asm/kprobes.h>
 #include <asm/stacktrace.h>
+#include <asm/nmi.h>
 
 #if defined(CONFIG_DEBUGGER) || defined(CONFIG_KEXEC_CORE)
 int (*__debugger)(struct pt_regs *regs) __read_mostly;