[release-branch.go1.11] crypto/x509: explicitly cast printf format argument

After CL 128056 the build fails on darwin/386 with

  src/crypto/x509/root_cgo_darwin.go:218:55: warning: values of type 'SInt32' should not be used as format arguments; add an explicit cast to 'int' instead [-Wformat]
  go build crypto/x509: C compiler warning promoted to error on Go builders

Fix the warning by explicitly casting the argument to an int as
suggested by the warning.

Fixes #30444

Change-Id: Icb6bd622a543e9bc5f669fd3d7abd418b4a8e579
Reviewed-on: https://go-review.googlesource.com/c/152958


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit ec0077c5)
Reviewed-on: https://go-review.googlesource.com/c/164240


Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>