BUILD: ssl: avoid mixed declarations in the AWS-LC curve fetch

BUILD: ssl: avoid mixed declarations in the AWS-LC curve fetch

The AWS-LC API 30-34 curve fetch declares the group variable after
statements, which fails builds using -Werror=declaration-after-statement.

Move the declaration to the beginning of smp_fetch_ssl_fc_ec() and keep
the assignment in the AWS-LC branch. Mark the variable __maybe_unused
for builds using other SSL libraries.

This fix must be backported wherever commit e671cb696d ("MINOR: ssl:
support curve fetches with AWS-LC API 30") was backported.