wgengine/netstack/gro: permit building without GRO

This only saves ~32KB in the minimal linux/amd64 binary, but it's a
step towards permitting not depending on gvisor for small builds.

Updates #17283

Change-Id: Iae8da5e9465127de354dbcaf25e794a6832d891b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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