fix error support clang

[sgzGary]

I meet problems like this when I use clang:
[ 172s] ssh-ecdsa.c:267:6: error: call to undeclared function 'is_ecdsa_pkcs11'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
[ 172s] 267 | if (is_ecdsa_pkcs11(key->ecdsa)) {
[ 172s] | ^
[ 172s] 1 warning and 1 error generated.
so i need to add head support

[djmdjm]

I don't know what you are doing but there is no function is_ecdsa_pkcs11() in OpenSSH. Maybe you are using some third-party patch? If so, you need to send this to the patch authors.