libc++: Install headers in include/, rather than lib/

When using the configure build system, the libc++ headers were being
installed in lib/, whereas cmake installs them in include/. Since
include/ makes more sense for headers, we'll make both systems install
headers there.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194833 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/runtime/libcxx/Makefile b/runtime/libcxx/Makefile
index 42322dc..a7df868 100644
--- a/runtime/libcxx/Makefile
+++ b/runtime/libcxx/Makefile
@@ -15,7 +15,7 @@
 CLANG_LEVEL := ../..
 include $(CLANG_LEVEL)/Makefile
 
-PROJ_libcxx_hdrs := $(DESTDIR)$(PROJ_prefix)/lib
+PROJ_libcxx_hdrs := $(DESTDIR)$(PROJ_prefix)/include
 
 # Expect libcxx to be in llvm/projects/libcxx
 LIBCXX_SRC_ROOT := $(LLVM_SRC_ROOT)/projects/libcxx