c_rehash: Fix file extension matching

For some reason, parenthesis were added 8 years ago in commit
a787c259. This essentially removed the
\. and $ constructs from the middle branches. Hence a file called e.g.
cert.key would accidentally match the (cer) rule.

CLA: trivial

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19427)