Fix build failure in benchmark in reduced configs

The "proper" fix would be to define the function only when it's needed,
but the condition for that would be tedious to write (enumeration of all
symmetric crypto modules) and since this is a utility program, not the
core library, I think it's OK to keep unused functions.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

programs/test/benchmark.c

@@ -643,6 +643,10 @@ int main( int argc, char *argv[] )
     memset( buf, 0xAA, sizeof( buf ) );
     memset( tmp, 0xBB, sizeof( tmp ) );
 
+    /* Avoid "unused static function" warning in configurations without
+     * symmetric crypto. */
+    (void) mbedtls_timing_hardclock( );
+
 #if defined(MBEDTLS_MD5_C)
     if( todo.md5 )
         TIME_AND_TSC( "MD5", mbedtls_md5( buf, BUFSIZE, tmp ) );