asahi/lib: generate git_sha1.h for agx_device.c

src/asahi/lib/agx_device.c includes git_sha1.h since 0be124b7 ("asahi: Deserialize libagx when opening device"), although it only started making use of it once ece3896d ("asahi: add broken bits of unstable Linux UAPI") was merged.

Regardless, without this meson change, this leads to a race condition in builds, where git_sha1.h might be built (if ever) after agx_device.c.