[test] Add missing cmake include for building libFuzzer alone

Include CompilerRTCompile in fuzzer tests explicitly.  Otherwise, when
building only libFuzzer, CMake fails due to:

CMake Error at cmake/Modules/AddCompilerRT.cmake:395 (sanitizer_test_compile):
  Unknown CMake command "sanitizer_test_compile".
Call Stack (most recent call first):
  lib/fuzzer/tests/CMakeLists.txt:53 (generate_compiler_rt_tests)

Differential Revision: https://reviews.llvm.org/D55378

llvm-svn: 348524