[PATCH] lockable.h: always define unknown_lock_type

From: Mingli Yu There comes below build failure when use gcc 10.1.0 and "-Og" passed to compiler. | /usr/lib/gcc/x86_64-wrs-linux/10.1.0/../../../../x86_64-wrs-linux/bin/ld.bfd: /mnt/build/tmp/work/x86_64-linux/qemu-system-native/5.1.0-r0/qemu-5.1.0/fsdev/qemu-fsdev-throttle.c:25: undefined r

[PATCH] lockable.h: always define unknown_lock_type

From: Mingli Yu There comes below build failure when use gcc 10.1.0 and "-Og" passed to compiler. | /usr/lib/gcc/x86_64-wrs-linux/10.1.0/../../../../x86_64-wrs-linux/bin/ld.bfd: /mnt/build/tmp/work/x86_64-linux/qemu-system-native/5.1.0-r0/qemu-5.1.0/fsdev/qemu-fsdev-throttle.c:25: undefined r