Fix compilation against Coq master.

1 job for master in 17 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
failed #31848
fp
build-iris.dev

00:17:46

 
Name Stage Failure
failed
build-iris.dev Build
Makefile.coq:320: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 2m6.492s
user 4m22.488s
sys 0m9.716s
ERROR: Job failed: exit code 1