Missing yypstate_clear prototype

I have hit a missing prototype warning for yypstate_clear() after
upgrading my bison installation to 3.6.2.

I have a reproducing example (not a particularly small one) in my
repository, by curling the following url:

https://git.sr.ht/~splintermail-dev/splintermail-client/blob/dev/libimap/parse.y

Then the generated file (bison parse.y -o parse.tab.c) has a
yypstate_clear function with no prototype.

Ryan