Move system/core/include from -isystem to -I

-isystem hides all warnings.  The warnings in system/core/include have
been cleaned up, so move it from -isystem to -I.

Test: lunch aosp_angler-eng && m -j
Test: lunch aosp_bullhead-eng && m -j
Test: lunch aosp_flounder-eng && m -j
Test: lunch aosp_fugu-eng && m -j
Test: lunch aosp_shamu-eng && m -j
Bug: 31492149
Change-Id: I29967428c2c45f753dabe21b65913fb08fdcabc4
diff --git a/cc/config/global.go b/cc/config/global.go
index 5b49bc3..d17a7d1 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -93,10 +93,11 @@
 	// Everything in these lists is a crime against abstraction and dependency tracking.
 	// Do not add anything to this list.
 	pctx.PrefixedPathsForOptionalSourceVariable("CommonGlobalIncludes", "-I",
-		[]string{})
-	pctx.PrefixedPathsForOptionalSourceVariable("CommonGlobalSystemIncludes", "-isystem ",
 		[]string{
 			"system/core/include",
+		})
+	pctx.PrefixedPathsForOptionalSourceVariable("CommonGlobalSystemIncludes", "-isystem ",
+		[]string{
 			"system/media/audio/include",
 			"hardware/libhardware/include",
 			"hardware/libhardware_legacy/include",