protobuf: allow compiling without protoc #1019

Job	Run time
Rustfmt	19s
Clippy	1m 12s
Check rustdoc intra-doc links	37s
Test Suite	1m 51s
Check	1m 43s
Cross compile (armv7-unknown-linux-gnueabihf)	21s
Cross compile (wasm32-unknown-unknown)	20s
Cross compile (powerpc-unknown-linux-gnu)	20s
Cross compile (powerpc64-unknown-linux-gnu)	18s
	7m 1s

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