Include Clang headers from ".../include" directory.

Do it with -isystem instead of -I.

Change-Id: I70fd55772e59a35f0b5a56816d61120f4921f4a4
diff --git a/core/llvm_config.mk b/core/llvm_config.mk
index ffa933d..d46f2c3 100644
--- a/core/llvm_config.mk
+++ b/core/llvm_config.mk
@@ -50,7 +50,7 @@
     -mbionic
 endif
 
-CLANG_CONFIG_EXTRA_C_INCLUDES := external/clang/lib/Headers
+CLANG_CONFIG_EXTRA_TARGET_C_INCLUDES := external/clang/lib/include
 
 # remove unknown flags to define CLANG_FLAGS
 TARGET_GLOBAL_CLANG_FLAGS += $(filter-out $(CLANG_CONFIG_UNKNOWN_CFLAGS),$(TARGET_GLOBAL_CFLAGS))