fix typo

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@50470 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/raster/null_val.c

@@ -257,7 +257,7 @@ int Rast_is_f_null_value(const FCELL * fcellVal)
    \return TRUE if DCELL raster value is NULL
    \return FALSE otherwise
  */
-#ifndef Rast_is_f_null_value
+#ifndef Rast_is_d_null_value
 int Rast_is_d_null_value(const DCELL * dcellVal)
 {
     return *dcellVal != *dcellVal;