gpuinfo: Make CGO optional on darwin

Signed-off-by: Piotr Stankiewicz <piotr.stankiewicz@docker.com>

pkg/gpuinfo/memory_darwin_cgo.go

@@ -1,3 +1,5 @@
+//go:build darwin && cgo
+
 package gpuinfo
 
 /*

pkg/gpuinfo/memory_darwin_nocgo.go

@@ -0,0 +1,10 @@
+//go:build darwin && !cgo
+
+package gpuinfo
+
+import "errors"
+
+// getVRAMSize returns total system GPU memory in bytes
+func getVRAMSize(_ string) (uint64, error) {
+	return 0, errors.New("unimplemented without cgo")
+}