Don't try to look for PLIST under .OBJDIR

If MAKEOBJDIRPREFIX and WRKDIRPREFIX are same PLIST may point to a
non-existing file under .OBJDIR without breaking build.

$ export MAKEOBJDIRPREFIX=/tmp
$ echo WRKDIRPREFIX=/tmp >>${__MAKE_CONF-/etc/make.conf}
$ cd /usr/ports/print/harfbuzz-icu
$ make clean patch
$ make -V .OBJDIR
/tmp/usr/ports/print/harfbuzz-icu
$ make install
$ pkg info -l harfbuzz-icu
harfbuzz-icu-1.5.1_2:

PR: 219008
Submitted by: Ilia Skalozubov (based on)
Approved by: portmgr blanket