gn2bp: add jni_headers if --sysroot is set in cflags

This is a hack to get jni targets that expect to get jni.h from sysroot
to work.
We should implement proper (whatever that means...) cflag parsing.

Test: //base:base
Change-Id: I8c1f99ea2087284eb1c2950071861db2913fe155
2 files changed