configure: fix SLIBNAME_WITH_MAJOR for openbsd (and bitrig in the next commit)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -2711,6 +2711,7 @@ case $target_os in
         enable pic
         disable symver
         SHFLAGS='-shared'
+        SLIBNAME_WITH_MAJOR='$(SLIBNAME).$(LIBVERSION)'
         oss_indev_extralibs="-lossaudio"
         oss_outdev_extralibs="-lossaudio"
         ;;