Fix compilation with Coq master.

The `[$]` pattern was used in an ambiguous way; there where multiple
hypotheses that could be used, and for some reason a different is
picked now.
1 job for master in 10 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #32187


Name Stage Failure
failed 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 0m38.131s
user 0m59.584s
sys 0m4.176s
ERROR: Job failed: exit code 1