cln-rpc/Makefile: fix typo CLN_RPC_GEN_ALL=>CLN_RPC_GENALL

[whitslack]

I noticed a typo in cln-rpc/Makefile while trying to hunt down why Make was remaking targets.

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[whitslack]

Also, shouldn't cln-grpc/Makefile be included from somewhere? It's not. Thus, make cln-grpc-all leads to…

make: *** No rule to make target 'cln-grpc-all'.  Stop.

It looks like include cln-grpc/Makefile should go inside the ifneq ($(RUST),0) block in the root Makefile.