powerpc/mce: Include prototypes

It fixes these W=1 compile errors :

../arch/powerpc/kernel/mce.c:591:14: error: no previous prototype for ‘machine_check_early’ [-Werror=missing-prototypes]
  591 | long notrace machine_check_early(struct pt_regs *regs)
      |              ^~~~~~~~~~~~~~~~~~~
../arch/powerpc/kernel/mce.c:725:6: error: no previous prototype for ‘hmi_exception_realmode’ [-Werror=missing-prototypes]
  725 | long hmi_exception_realmode(struct pt_regs *regs)
      |      ^~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-8-clg@kaod.org

arch/powerpc/kernel/mce.c

@@ -21,6 +21,7 @@
 #include <asm/machdep.h>
 #include <asm/mce.h>
 #include <asm/nmi.h>
+#include <asm/asm-prototypes.h>
 
 static DEFINE_PER_CPU(int, mce_nest_count);
 static DEFINE_PER_CPU(struct machine_check_event[MAX_MC_EVT], mce_event);