Mgv6: Add fallback node for gravel

Gravel now falls back to stone.
Gravel is not a particularly fundamental node, allowing it to fall back
to stone frees up subgames from having to include a gravel node.
Non-blob-ore gravel is only present in mgv6 as extremely rare 'gravel
biomes'.

src/mapgen_v6.cpp

@@ -99,6 +99,8 @@ MapgenV6::MapgenV6(int mapgenid, MapgenV6Params *params, EmergeManager *emerge)
 	c_snowblock       = ndef->getId("mapgen_snowblock");
 	c_ice             = ndef->getId("mapgen_ice");
 
+	if (c_gravel == CONTENT_IGNORE)
+		c_gravel = c_stone;
 	if (c_desert_stone == CONTENT_IGNORE)
 		c_desert_stone = c_stone;
 	if (c_desert_sand == CONTENT_IGNORE)