Use -isystem to export ndk stl includes

Hides:
In file included from frameworks/rs/cpu_ref/rsCpuExecutable.cpp:4:
In file included from prebuilts/ndk/current/sources/cxx-stl/llvm-libc++/include/fstream:169:
In file included from prebuilts/ndk/current/sources/cxx-stl/llvm-libc++/include/ostream:138:
In file included from prebuilts/ndk/current/sources/cxx-stl/llvm-libc++/include/ios:216:
In file included from prebuilts/ndk/current/sources/cxx-stl/llvm-libc++/include/__locale:21:
prebuilts/ndk/current/sources/android/support/include/locale.h:97:9: error: 'LC_GLOBAL_LOCALE' macro redefined [-Werror,-Wmacro-redefined]
       ^
out-soong/ndk/sysroot/usr/include/locale.h:115:9: note: previous definition is here
       ^

Test: builds
Change-Id: I766e8645db33e6e71f44a131aa0b1649d83e8c51
diff --git a/cc/ndk_prebuilt.go b/cc/ndk_prebuilt.go
index f1bd3b5..15a9d28 100644
--- a/cc/ndk_prebuilt.go
+++ b/cc/ndk_prebuilt.go
@@ -190,7 +190,7 @@
 		ctx.ModuleErrorf("NDK prebuilts must have an ndk_lib prefixed name")
 	}
 
-	ndk.exportIncludes(ctx, "-I")
+	ndk.exportIncludes(ctx, "-isystem")
 
 	libName := strings.TrimPrefix(ctx.ModuleName(), "ndk_")
 	libExt := flags.Toolchain.ShlibSuffix()