scripts/build: make struct_tags.json deterministic

It's not good to see struct_tags.json change from one build to the next
when nothing changes.

Python's sets are not deterministic, see long story for older commit
f896fc23 ("scripts: gen_handles: Sort the device handles")

Simply convert multiple_directories to a sorted list before using it.

Fixes commit 80e78208 ("kernel: syscalls: no need to include all
syscalls in binary")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>