Disable Qt4 sql-ibase plugin on not-(i386|amd64)

ibase is Firebird, which only builds on i386 and amd64. This patch
squashes the ibase plugin for Qt4.

PR: 216943
Submitted by: linimon
Reported by: linimon
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D14326