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_os_SharedMemory.cpp b/core/jni/android_os_SharedMemory.cpp
index 1d29908..f6e5c7a 100644
--- a/core/jni/android_os_SharedMemory.cpp
+++ b/core/jni/android_os_SharedMemory.cpp
@@ -20,7 +20,7 @@
 
 #include <cutils/ashmem.h>
 #include <utils/Log.h>
-#include "JNIHelp.h"
+#include <nativehelper/JNIHelp.h>
 #include <nativehelper/JniConstants.h>
 #include <nativehelper/ScopedLocalRef.h>