ec: Use static linkage on nistp521 felem_{square,mul} wrappers

ec: Use static linkage on nistp521 felem_{square,mul} wrappers

Runtime selection of implementations for felem_{square,mul} depends on
felem_{square,mul}_wrapper functions, which overwrite function points in
a similar design to that of .plt.got sections used by program loaders
during dynamic linking.

There's no reason why these functions need to have external linkage.
Mark static.

Signed-off-by: Rohan McLure <rohanmclure@linux.ibm.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/21471)