net/http/cgi: fix plan9 build
net/http/cgi: fix plan9 build

Cleanup CL https://golang.org/cl/41691 broke the plan9 build by removing
a use of a package but not removing the package import.

Trybots don't check that. I filed #20119 for that.

Change-Id: Ia030e6924665dfb871ca964455b899d51b0200c2
Reviewed-on: https://go-review.googlesource.com/41752
Reviewed-by: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>

polyverse-security/go,cloudflare/go,CAFxX/go,derekmarcotte/go,cloudflare/go,Cofyc/go,golang/go,akutz/go,polyverse-security/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,derekmarcotte/go,nawawi/go,golang/go,ImJasonH/go,derekmarcotte/go,golang/go,ImJasonH/go,golang/go,vsekhar/elastic-go,codestation/go,vsekhar/elastic-go,golang/go,Cofyc/go,CAFxX/go,cloudflare/go,codestation/go,nawawi/go,Cofyc/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,Cofyc/go,josharian/go,cloudflare/go,codestation/go,nawawi/go,josharian/go,Cofyc/go,akutz/go,akutz/go,Cofyc/go,codestation/go,golang/go,josharian/go,cloudflare/go,Cofyc/go,vsekhar/elastic-go,codestation/go,codestation/go,CAFxX/go,derekmarcotte/go,ImJasonH/go,ImJasonH/go,akutz/go,Cofyc/go,vsekhar/elastic-go,josharian/go,golang/go,polyverse-security/go,josharian/go,ImJasonH/go,codestation/go,polyverse-security/go,codestation/go,akutz/go,nawawi/go,polyverse-security/go,josharian/go,derekmarcotte/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,ImJasonH/go,golang/go,derekmarcotte/go,nawawi/go,josharian/go,derekmarcotte/go,akutz/go,polyverse-security/go,polyverse-security/go,derekmarcotte/go,vsekhar/elastic-go,christopher-henderson/Go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,ImJasonH/go