audio/wildmidi: build fix for an undeclared identifier error.

[ultima]

Summary

This patch will fix BROKEN status on several latest
FreeBSD branches that will be upcoming releases.

PR\: 220843
Submitted by\: SimaMoto,RyōTa <[e-mail]> (maintainer)
Reviewed by\: lifanov (mentor), matthew (mentor)
Approved by\: lifanov (mentor), matthew (mentor)
MFH: 2017Q3
Differential Revision\: https://reviews.freebsd.org/DXXXXX

This shouldn't require revision bump as it is to only fix broken versions of FreeBSD.

Test Plan

portlint:
WARN: Makefile: [14]: USE_* seen before USES. According to the porters-handbook, USES must appear first.
WARN: Makefile: for new port, make $FreeBSD$ tag in comment section empty, to make SVN happy.
WARN: Makefile: "LICENSE_COMB" has to appear earlier.
0 fatal errors and 3 warnings found.

Item #1 will be fixed in next patch which will be update.

poudriere:
103i386
103amd64
110i386
110amd64
12i386
12amd64

[matthew]

Is this update still needed given D11831?

[matthew]

Ah, I see this is independent of D11831.

I think this can be committed straight away.