libandroid_runtime: android_print needs NELEM def

NELEM is defined many places in the framework, so this gives
android_print_PdfDocument.cpp one of them (JNIHelp.h).  Without this
updating an upstream library (libskia) to remove an NELEM definition
causes this file to break.

Change-Id: Ifd033f9bd92b8902b6035ec5a8381cb9debd87ce