c-index-test/CMakeLists.txt: Suggest -isystem to include libxml2 with include_directories(SYSTEM).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185928 91177308-0d34-0410-b5e6-96231b3b80d8

tools/c-index-test/CMakeLists.txt

@@ -24,6 +24,6 @@ set_target_properties(c-index-test
 
 # If libxml2 is available, make it available for c-index-test.
 if (CLANG_HAVE_LIBXML)
-  include_directories(${LIBXML2_INCLUDE_DIR})
+  include_directories(SYSTEM ${LIBXML2_INCLUDE_DIR})
   target_link_libraries(c-index-test ${LIBXML2_LIBRARIES})
 endif()