emulators/wine-devel: Change tested variable for master port detection

It has been observed that the target config-recursive for
emulators/i386-wine-devel can cause bsd.port.mk to be included twice when
run in i386 jails. This seems to be due to the fact that config recursive
ignores PKGNAMEPREFIX definiton, which is used in emulators/wine-devel to
test whether bsd.port.[pre.|post.]mk needs to be included or bsd.port.mk
has already been included by i386-wine-devel. We change the testing
variable to WINE_SLAVE_BUILD, defined in i386-wine-devel/Makefile.i386.

Approved by: gerald (maintainer, mentor)
Differential Revision: https://reviews.freebsd.org/D24167