gpu/internal/vulkan: support novulkan tag, FreeBSD

The novulkan tag didn't completely disable the Vulkan backend, and
the backend wouldn't register on FreeBSD. This change fixes both
issues.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/vulkan/vulkan.go

@@ -1,7 +1,8 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-//go:build linux
-// +build linux
+//go:build (linux || freebsd) && !novulkan
+// +build linux freebsd
+// +build !novulkan
 
 package vulkan