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.
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index be7f41b..e76f4f8 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -766,6 +766,9 @@
 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 )
 {
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index fd69763..ac652d2 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -279,8 +279,8 @@
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
-#if defined(MBEDTLS_SSL_CLI_C)
-static int ssl_session_copy( mbedtls_ssl_session *dst, const mbedtls_ssl_session *src )
+int mbedtls_ssl_session_copy( mbedtls_ssl_session *dst,
+                              const mbedtls_ssl_session *src )
 {
     mbedtls_ssl_session_free( dst );
     memcpy( dst, src, sizeof( mbedtls_ssl_session ) );
@@ -319,7 +319,6 @@
 
     return( 0 );
 }
-#endif /* MBEDTLS_SSL_CLI_C */
 
 #if defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
 int (*mbedtls_ssl_hw_record_init)( mbedtls_ssl_context *ssl,
@@ -7613,7 +7612,8 @@
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
     }
 
-    if( ( ret = ssl_session_copy( ssl->session_negotiate, session ) ) != 0 )
+    if( ( ret = mbedtls_ssl_session_copy( ssl->session_negotiate,
+                                          session ) ) != 0 )
         return( ret );
 
     ssl->handshake->resume = 1;
@@ -8548,7 +8548,7 @@
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
     }
 
-    return( ssl_session_copy( dst, ssl->session ) );
+    return( mbedtls_ssl_session_copy( dst, ssl->session ) );
 }
 #endif /* MBEDTLS_SSL_CLI_C */