Make sure install Clang headers when building clang-cc

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74163 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Headers/CMakeLists.txt b/lib/Headers/CMakeLists.txt
index 318685a..57d7ee5 100644
--- a/lib/Headers/CMakeLists.txt
+++ b/lib/Headers/CMakeLists.txt
@@ -31,7 +31,7 @@
     COMMENT "Copying clang's ${f}...")
 endforeach( f )
 
-add_custom_target(clang_headers ALL
+add_custom_target(clang-headers ALL
   DEPENDS ${files})
 
 install(FILES ${files}