[clang][deps] Explicitly link pthreads

This is a speculative fix for link failure found by the RHEL 8 bot: https://lab.llvm.org/buildbot/#/builders/204/builds/3862.

FAILED: lib/libclangDependencyScanning.so.21.0git
/InProcessModuleCache.cpp.o: In function `(anonymous namespace)::ReaderWriterLock::tryLock()':
InProcessModuleCache.cpp:(.text._ZN12_GLOBAL__N_116ReaderWriterLock7tryLockEv+0x1d): undefined reference to `pthread_rwlock_trywrlock'

(cherry picked from commit 15ad84f)

Author: jansvoboda11

clang/lib/Tooling/DependencyScanning/CMakeLists.txt

@@ -30,4 +30,5 @@ add_clang_library(clangDependencyScanning
   clangLex
   clangSerialization
   clangTooling
+  ${LLVM_PTHREAD_LIB}
   )