Makefile: ensure that cln-grpc depends on msggen generated files.

Looks like a build race, where we use it before it's finished:

```
msggen cln-rpc/src/model.rs
error: this file contains an unclosed delimiter
    --> cln-grpc/src/server.rs:3661:7
     |
84   | {
     | - unclosed delimiter
...
3661 |     }
     |      ^
```
...

```
make: *** [plugins/Makefile:298: target/release/cln-grpc] Error 101
make: *** Waiting for unfinished jobs....
```

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

plugins/Makefile

@@ -294,7 +294,7 @@ CLN_PLUGIN_EXAMPLES := \
 CLN_PLUGIN_SRC = $(shell find plugins/src -name "*.rs")
 CLN_GRPC_PLUGIN_SRC = $(shell find plugins/grpc-plugin/src -name "*.rs")
 
-target/${RUST_PROFILE}/cln-grpc: ${CLN_PLUGIN_SRC} ${CLN_GRPC_PLUGIN_SRC}
+target/${RUST_PROFILE}/cln-grpc: ${CLN_PLUGIN_SRC} ${CLN_GRPC_PLUGIN_SRC} $(MSGGEN_GEN_ALL)
 	cargo build ${CARGO_OPTS} --bin cln-grpc
 
 ifneq ($(RUST),0)