vim: prevent binaries from being stripped too early

By default, vim binaries are stripped during the build. This prevents us from
putting non-stripped binaries into a -dbg package. Passing 'STRIP=/bin/true' to
configure is the method recommended in the vim Makefile to disable stripping of
binaries during the build.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>