protobuf: allow compiling without `protoc` #4

Job	Run time
Check	1m 38s
Test Suite	2m 2s
Cross compile (powerpc64-unknown-linux-gnu)	20s
Cross compile (armv7-unknown-linux-gnueabihf)	20s
Rustfmt	24s
Check rustdoc intra-doc links	36s
Cross compile (powerpc-unknown-linux-gnu)	20s
Clippy	1m 19s
Cross compile (wasm32-unknown-unknown)	20s
	7m 19s

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