[foreign] RFR: undo changes to include aux paths in TestJextractFFI

[Maurizio Cimadamore]

Hi,
sometimes ago, I added some changes to the TestJextractFFI to pick up 
some auxiliary include files that were necessary on linux.

These changes are not required after Sundar's related push [1].

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/clang_ffi_undo_aux/

Maurizio

[1] - 
http://mail.openjdk.java.net/pipermail/panama-dev/2018-July/002252.html