Define a value for SYS_F_FCNTL

This symbol was added in commit d33b215b331116e50947ca7e75d210e1db39b78d
but was only used in certain (presumed uncommon) preprocessor conditionals,
as no build failures have been reported yet.

Reported by Balaji Marisetti.

Closes: #4029

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4030)