contrib: rav1e: fix parallel build (fixes #26518)

Newer versions of GNU/make mark the job server pipe file descriptors
with the close-on-exec flag. This prevents the file descriptors from
being leaked into other programs executed by the children processes
of GNU/make.

But of course, the flip side is that the file descriptors are closed
even if the program is also a make-like tool that wants to participate
in the job counting and access the job server.

To support recursive make, GNU/make will clear the close-on-exec flag
in a child process that is about to execute a command:
- referencing the variable MAKE, or
- prefixed by a plus sign '+'.

In this particular case, the command is $(CARGO_VENDOR_SETUP), not
$(MAKE), so this does not work out of the box. Add an explicit plus.

contrib/src/rav1e/rules.mak

@@ -29,5 +29,5 @@ rav1e: rav1e-$(RAV1E_VERSION).tar.gz .sum-rav1e .rav1e-vendor
 	$(MOVE)
 
 .rav1e: rav1e .cargo
-	cd $< && $(CARGOC_INSTALL) --no-default-features $(RAV1E_FEATURES)
+	+cd $< && $(CARGOC_INSTALL) --no-default-features $(RAV1E_FEATURES)
 	touch $@