lightningd: mangle parameter names in bitcoind_getrawblockbyheight_ so mocks don't get shadow warning.

When we make mocks (which the next patch will do), these names cause a warning:

```
wallet/test/run-db.c:32:64: error: declaration of ‘bitcoind’ shadows a parameter [-Werror=shadow=compatible-local]
   32 |                                    void (*cb)(struct bitcoind *bitcoind UNNEEDED,
wallet/test/run-db.c:30:53: note: shadowed declaration is here
   30 |                                    struct bitcoind *bitcoind UNNEEDED,
wallet/test/run-db.c:33:51: error: declaration of ‘height’ shadows a parameter [-Werror=shadow=compatible-local]
   33 |                                               u32 height UNNEEDED,
wallet/test/run-db.c:31:40: note: shadowed declaration is here
   31 |                                    u32 height UNNEEDED,
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

lightningd/bitcoind.h

@@ -127,8 +127,8 @@ void bitcoind_getchaininfo_(const tal_t *ctx,
 void bitcoind_getrawblockbyheight_(const tal_t *ctx,
 				   struct bitcoind *bitcoind,
 				   u32 height,
-				   void (*cb)(struct bitcoind *bitcoind,
-					      u32 height,
+				   void (*cb)(struct bitcoind *bitcoind_,
+					      u32 height_,
 					      struct bitcoin_blkid *blkid,
 					      struct bitcoin_block *blk,
 					      void *arg),