fpsensor: Enable -Wvla
fpsensor: Enable -Wvla

Ideally this should be enabled for all the EC code, but that requires
making more changes. For now enable it for the fingerprint sensor code
to make sure we don't accidentally introduce variable length arrays.

BRANCH=none
BUG=b:172020413
TEST=make buildall

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I80fe4acade2c84886142ef795e54be004d1663be
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2792699
Commit-Queue: Yicheng Li <77a205905d8955597c069555e9569c30512f6bd8@chromium.org>
Reviewed-by: Yicheng Li <77a205905d8955597c069555e9569c30512f6bd8@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec