bind/objc: fix benchmark
bind/objc: fix benchmark

The generator recently changed the default ObjC prefix to the empty
string but the benchmark wasn't changed accordingly (the benchmark
doesn't run in -short mode).

Fixes golang/go#19048

Change-Id: Id8f63da787f399410ea86c13ee77eb3005d31f38
Reviewed-on: https://go-review.googlesource.com/37314
Reviewed-by: Andris Valums <bc7efc38b43a19cddc642776c52682dff5b443f1@fastmail.com>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile