riscv: traps: add missing prototype

Sparse complains:
arch/riscv/kernel/traps.c:213:6: warning: symbol 'shadow_stack' was not declared. Should it be static?

The variable is used in entry.S, so declare shadow_stack there
alongside SHADOW_OVERFLOW_STACK_SIZE.

Fixes: 31da94c2 ("riscv: add VMAP_STACK overflow detection")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220814141237.493457-5-mail@conchuod.ieSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/thread_info.h

@@ -42,6 +42,8 @@
 
 #ifndef __ASSEMBLY__
 
+extern long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE / sizeof(long)];
+
 #include <asm/processor.h>
 #include <asm/csr.h>
 

arch/riscv/kernel/traps.c

@@ -20,9 +20,10 @@
 
 #include <asm/asm-prototypes.h>
 #include <asm/bug.h>
+#include <asm/csr.h>
 #include <asm/processor.h>
 #include <asm/ptrace.h>
-#include <asm/csr.h>
+#include <asm/thread_info.h>
 
 int show_unhandled_signals = 1;