RFR JDK-8196987: Resolve disabled warnings for libzip

[Xueming Shen]

Hi

Please help review the change for JDK-8196987.

issue: https://bugs.openjdk.java.net/browse/JDK-8196987
webrev: http://cr.openjdk.java.net/~sherman/8196987/webrev

It appears the change for JDK-6341887 in Deflater.c#169 "accidentally" 
fixed the offending
"implicit-fallthrough" warning/error [1]

[1] 
http://cr.openjdk.java.net/~sherman/6341887.David.Lloyd/webrev/src/java.base/share/native/libzip/Deflater.c.sdiff.html

So the proposed change is to simply restore/enable the warning_as_error. 
Verified with
mach5 job.

Thanks,
Sherman