Fix netstack build error on non-AMD64.

This stub had the wrong function signature.

PiperOrigin-RevId: 262992682
diff --git a/tcpip/link/fdbased/mmap.go b/tcpip/link/fdbased/mmap.go
index 9b1ae2c..2dca173 100644
--- a/tcpip/link/fdbased/mmap.go
+++ b/tcpip/link/fdbased/mmap.go
@@ -16,10 +16,8 @@
 
 package fdbased
 
-import "github.com/google/netstack/tcpip"
-
 // Stubbed out version for non-linux/non-amd64 platforms.
 
-func newPacketMMapDispatcher(fd int, e *endpoint) (linkDispatcher, *tcpip.Error) {
+func newPacketMMapDispatcher(fd int, e *endpoint) (linkDispatcher, error) {
 	return nil, nil
 }