microblaze: Fix sparse warning - sw_exceptions

Function sw_exception is linked with asm code.

Warning log:
CHECK   arch/microblaze/kernel/exceptions.c
arch/microblaze/kernel/exceptions.c:53:6: warning: symbol 'sw_exception' was not declared. Should it be static?

Signed-off-by: Michal Simek <monstr@monstr.eu>