fdt: Explicitly mark fdt_slicer as dependent on geom_flashmap

Without this dependency relationship, the linker doesn't find the
flash_register_slicer() function, so kldload fails to load fdt_slicer.ko.

Discussed with:	ian@

sys/dev/fdt/fdt_slicer.c

@@ -182,4 +182,5 @@ static moduledata_t fdt_slicer_mod = {
 };
 
 DECLARE_MODULE(fdt_slicer, fdt_slicer_mod, SI_SUB_DRIVERS, SI_ORDER_SECOND);
+MODULE_DEPEND(fdt_slicer, g_flashmap, 0, 0, 0);
 MODULE_VERSION(fdt_slicer, 1);