RFR(xs): 8078628: linux-zero does not build without precompiled header

[Thomas Stüfe]

Hi all,

please review this tiny change. Build was broken for zero/slowdebug when
building without precompiled header.

bug:       https://bugs.openjdk.java.net/browse/JDK-8078628
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8078628/webrev.00/webrev/


Thank you,

Thomas