typo: protc is not found

[kawanet]

This PR is to fix a typo in a message in configure.

The WARNING sounds weak and looks not enough as protoc is required to build tkrzw-rpc from source, by the way.

[estraier]

Thanks for the fix.
I agree that WARNING sound weak. Stopping the configure on the spot is a feasible option.
The reason I do this currently is that stopping by one issue prevents us from listing up all issues at once.
And for some people, fixing the problem by editing Makefile manually is easier than setting proper environment variables for configure.
So, let us keep this structure until we find a better solution.