Give ssl_session_copy() external linkage

A subsequent commit will need this function in the session ticket
and session cache implementations. As the latter are server-side,
this commit also removes the MBEDTLS_SSL_CLI_C guard.

For now, the function is declared in ssl_internal.h and hence not
part of the public API.

include/mbedtls/ssl_internal.h

@@ -766,6 +766,9 @@ int mbedtls_ssl_dtls_replay_check( mbedtls_ssl_context *ssl );
 void mbedtls_ssl_dtls_replay_update( mbedtls_ssl_context *ssl );
 #endif
 
+int mbedtls_ssl_session_copy( mbedtls_ssl_session *dst,
+                              const mbedtls_ssl_session *src );
+
 /* constant-time buffer comparison */
 static inline int mbedtls_ssl_safer_memcmp( const void *a, const void *b, size_t n )
 {