Tor fails to build on Ubuntu Groovy Gorilla: -Wextra-semi only used for Obj-C and C++

@weasel reported the following error today on #tor-dev: https://jenkins.torproject.org/job/tor-ci-linux-master/4880/

The error seems to be:

cc1: error: command-line option '-Wextra-semi' is valid for C++/ObjC++ but not for C [-Werror]
cc1: all warnings being treated as errors

As seen in: https://jenkins.torproject.org/job/tor-ci-linux-master/4880/ARCHITECTURE=amd64,SUITE=groovy/consoleText

We should probably disable -Wextra-semi as this seems to only apply for C++ and Objective C.