Link libpthread into LLVMCore.so

After r369018 the compiler can inline pthread calls into users of
RWMutex.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369029 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/CMakeLists.txt

@@ -61,6 +61,8 @@ add_llvm_library(LLVMCore
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/IR
 
+  LINK_LIBS ${LLVM_PTHREAD_LIB}
+
   DEPENDS
   intrinsics_gen
   )