compiler warning in pgcrypto imath.c

When compiling on an AWS 64 bit Arm machine, I get this compiler warning:

imath.c: In function 's_ksqr':
imath.c:2590:6: warning: variable 'carry' set but not used
[-Wunused-but-set-variable]
      carry;
      ^~~~~

With this version():

PostgreSQL 12devel on aarch64-unknown-linux-gnu, compiled by gcc
(Ubuntu/Linaro 7.3.0-27ubuntu1~18.04) 7.3.0, 64-bit

The attached patch adds PG_USED_FOR_ASSERTS_ONLY to silence it.  Perhaps
there is a better way, given that we want to change imath.c as little as
possible from its upstream?

Cheers,

Jeff