RISC-V: stacktrace: Declare sp_in_global outside ifdef

riscv:allnoconfig and riscv:tinyconfig fail to compile.

arch/riscv/kernel/stacktrace.c: In function 'walk_stackframe':
arch/riscv/kernel/stacktrace.c:78:8: error: 'sp_in_global' undeclared

sp_in_global is declared inside CONFIG_FRAME_POINTER but used outside
of it.

Fixes: 52e7c52d ("RISC-V: Stop relying on GCC's register allocator's hueristics")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>