cpp: use proper nativehelper headers

libnativehelper exports headers under nativehelper. These were
available before incorrectly as global headers in order to give
access to jni.h.

Test: modules using cpp find headers
Bug: 63762847
Change-Id: I34722bb961327b0128703ab925bc86961ca66cb6
diff --git a/core/jni/android_hardware_display_DisplayViewport.cpp b/core/jni/android_hardware_display_DisplayViewport.cpp
index 2d2837c..23c3877 100644
--- a/core/jni/android_hardware_display_DisplayViewport.cpp
+++ b/core/jni/android_hardware_display_DisplayViewport.cpp
@@ -16,7 +16,7 @@
 
 #define LOG_TAG "DisplayViewport-JNI"
 
-#include "JNIHelp.h"
+#include <nativehelper/JNIHelp.h>
 #include "core_jni_helpers.h"
 
 #include <android_hardware_display_DisplayViewport.h>