Export system headers for SdkVariants as well

This code block exists for LLDNK code path (m.UseVndk()...), but was
missing from NDK code path (m.IsSdkVariant())

Test: TH
Change-Id: Ib156d14a6f013c88499a0641dbb547c632357362
diff --git a/cc/library_stub.go b/cc/library_stub.go
index d21df51..08a5eb6 100644
--- a/cc/library_stub.go
+++ b/cc/library_stub.go
@@ -205,6 +205,14 @@
 				d.libraryDecorator.flagExporter.Properties.Export_include_dirs = append(
 					d.libraryDecorator.flagExporter.Properties.Export_include_dirs,
 					variantMod.exportProperties.Export_include_dirs...)
+
+				// Export headers as system include dirs if specified. Mostly for libc
+				if Bool(variantMod.exportProperties.Export_headers_as_system) {
+					d.libraryDecorator.flagExporter.Properties.Export_system_include_dirs = append(
+						d.libraryDecorator.flagExporter.Properties.Export_system_include_dirs,
+						d.libraryDecorator.flagExporter.Properties.Export_include_dirs...)
+					d.libraryDecorator.flagExporter.Properties.Export_include_dirs = nil
+				}
 			}
 		}
 	}