microblaze: Fix missing microblaze specific syscalls declaration

Warning log:
CHECK   arch/microblaze/kernel/sys_microblaze.c
arch/microblaze/kernel/sys_microblaze.c:37:17: warning: symbol 'microblaze_vfork' was not declared. Should it be static?
arch/microblaze/kernel/sys_microblaze.c:43:17: warning: symbol 'microblaze_clone' was not declared. Should it be static?
arch/microblaze/kernel/sys_microblaze.c:50:17: warning: symbol 'microblaze_execve' was not declared. Should it be static?

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