Fix bolt build with -DBUILD_SHARED_LIBS=ON after 71fbbb69

Commit 71fbbb69 moved getGUID out of
line in llvm/IR/GlobalValue, now users have to link LLVMCore to have
the definition of it.

/usr/bin/ld: CMakeFiles/LLVMBOLTRewrite.dir/PseudoProbeRewriter.cpp.o: in function `(anonymous namespace)::PseudoProbeRewriter::parsePseudoProbe()':
PseudoProbeRewriter.cpp:(.text._ZN12_GLOBAL__N_119PseudoProbeRewriter16parsePseudoProbeEv+0x3d0): undefined reference to `llvm::GlobalValue::getGUID(llvm::StringRef)'
/usr/bin/ld: CMakeFiles/LLVMBOLTRewrite.dir/PseudoProbeRewriter.cpp.o: in function `(anonymous namespace)::PseudoProbeRewriter::encodePseudoProbes()':
PseudoProbeRewriter.cpp:(.text._ZN12_GLOBAL__N_119PseudoProbeRewriter18encodePseudoProbesEv+0x11a1): undefined reference to `llvm::GlobalValue::getGUID(llvm::StringRef)'
collect2: error: ld returned 1 exit status
make[2]: *** [tools/bolt/lib/Rewrite/CMakeFiles/LLVMBOLTRewrite.dir/build.make:275: lib/libLLVMBOLTRewrite.so.19.0git] Error 1