Fix build with shared libraries

/usr/bin/ld: CMakeFiles/ClangReplInterpreterTests.dir/InterpreterExtensi
onsTest.cpp.o: undefined reference to symbol '_ZN4llvm14TargetRegistry12
lookupTargetENS_9StringRefERNSt7__cxx1112basic_stringIcSt11char_traitsIc
ESaIcEEE'
/usr/bin/ld: /work/kparzysz/git/llvm.org/b/x86/lib/libLLVMMC.so.19.0git:
 error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

The missing symbol is `llvm::TargetRegistry::lookupTarget`, which
interestingly enough is in MC.

Add `MC` to the list of LLVM dependencies.