BUILD: ssl: fix build error on older compilers with openssl-3.2

BUILD: ssl: fix build error on older compilers with openssl-3.2

OpenSSL 3.2 triggers the code part added by commit 25da217 ("MINOR: ssl:
Update ssl_fc_curve/ssl_bc_curve to use SSL_get0_group_name") which
contains a variable declaration in the for() statement and breaks on
older compilers, as reported in GH issues #2501.

Let's just declare it normally to fix the problem. This must be
backported wherever the commit above is (at least 2.9).