Fix missing includes in `fmpi_task_generic.h`

include/internal/generic/fmpi_task_generic.h

@@ -26,6 +26,9 @@
 /*==============================================================================
     INCLUDE
 ==============================================================================*/
+// C Standard Library
+#include <assert.h>
+#include <stddef.h> // NULL
 // External
 #include "../../../external/cpl/cpl_arg.h"
 #include "../../../external/cpl/cpl_map.h"