contrib: build cmake targets in the same job pool as the other targets

I believe that's what the + is for. It's used on all other $(CMAKEBUILD)
calls.

contrib/src/libaribcaption/rules.mak

@@ -79,5 +79,5 @@ endif
 .libaribcaption: libaribcaption toolchain.cmake
 	cd $< && rm -f CMakeCache.txt
 	cd $< && $(HOSTVARS_PIC) $(CMAKE) $(LIBARIBCC_CONF)
-	$(CMAKEBUILD) $< --target install
+	+$(CMAKEBUILD) $< --target install
 	touch $@

contrib/src/vulkan-loader/rules.mak

@@ -56,7 +56,7 @@ VULKAN_LOADER_ENV_CONF = \
 	cd $< && rm -rf ./build && mkdir -p build
 	cd $</build && $(VULKAN_LOADER_ENV_CONF) $(HOSTVARS) \
 		$(CMAKE) $(VULKAN_LOADER_CONF) ..
-	$(CMAKEBUILD) $</build
+	+$(CMAKEBUILD) $</build
 
 ifdef HAVE_WIN32
 # CMake will generate a .pc file with -lvulkan even if the static library