net/citrix_ica: Add webplugin USES flag

[xmj]

Summary

As reported in PR 179420 a previous change to net/citrix_ica broke its
integration into common webbrowsers.
Fix this by adding a webplugin:linux USES flag.

While there, clean up plist.

net/citrix_ica: Add webplugin USES flag

- Add USES= webplugin:linux
- Plist cleanup

PR: 			179420
DiffRev: 		(placeholder)
Submitted by: 	[e-mail]
Approved by:		swills (mentor)

Test Plan

http://xmj.me/freebsd/buildlogs/citrix_ica-12.1_2.log

[swills]

Approved

[xmj]

Committed revision 372265.