powerpc/32: Declare stack_overflow_exception() prototype

This fixes a compile error with W=1.

CC      arch/powerpc/kernel/traps.o
../arch/powerpc/kernel/traps.c:1663:6: error: no previous prototype for ‘stack_overflow_exception’ [-Werror=missing-prototypes]
 void stack_overflow_exception(struct pt_regs *regs)
      ^~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 3978eb7 ("powerpc/32: Add early stack overflow detection with VMAP stack.")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200914211007.2285999-8-clg@kaod.org

arch/powerpc/include/asm/asm-prototypes.h

@@ -67,6 +67,7 @@ void single_step_exception(struct pt_regs *regs);
 void program_check_exception(struct pt_regs *regs);
 void alignment_exception(struct pt_regs *regs);
 void StackOverflow(struct pt_regs *regs);
+void stack_overflow_exception(struct pt_regs *regs);
 void kernel_fp_unavailable_exception(struct pt_regs *regs);
 void altivec_unavailable_exception(struct pt_regs *regs);
 void vsx_unavailable_exception(struct pt_regs *regs);