runtime: correct GoCheckBindM's C declaration in EnsureBindM test

runtime: correct GoCheckBindM's C declaration in EnsureBindM test

The test file has a C declaration which doesn't match the actual
definition. Remove it and include "_cgo_export.h" to have the
right declaration.

Change-Id: Iddf6d8883ee0e439147c7027029dd3e352ef090d
Reviewed-on: https://go-review.googlesource.com/c/go/+/482975
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>