contrib: fail the build if prebuilt fail

In some cases the tarballs are corrupted. If the extraction of the tarball
fails the build will fail (or may use part of an old extraction).

The `-` was introduced in 5d2ed11e.

contrib/src/main.mak

@@ -562,7 +562,7 @@ vlc-contrib-$(HOST)-latest.tar.bz2:
 
 prebuilt: vlc-contrib-$(HOST)-latest.tar.bz2
 	$(RM) -r $(PREFIX)
-	-$(UNPACK)
+	$(UNPACK)
 	mv $(HOST) $(PREFIX)
 	cd $(PREFIX) && $(abspath $(SRC))/change_prefix.sh
 ifdef HAVE_WIN32