stupidgcm: add BenchmarkCCall

gocryptfs/internal/stupidgcm$ go test -bench .
goos: linux
goarch: amd64
pkg: github.com/rfjakob/gocryptfs/v2/internal/stupidgcm
cpu: Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz
BenchmarkCCall-4   	15864030	        78.60 ns/op
PASS
ok  	github.com/rfjakob/gocryptfs/v2/internal/stupidgcm	1.898s

1 files changed, 6 insertions, 0 deletions

diff --git a/internal/stupidgcm/openssl.go b/internal/stupidgcm/openssl.go
index cb9845e..82ec0a1 100644
--- a/internal/stupidgcm/openssl.go
+++ b/internal/stupidgcm/openssl.go
@@ -108,3 +108,9 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 	}
 	return append(dst, buf...), nil
 }
+
+// This functions exists to benchmark the C call overhead from Go.
+// See BenchmarkCCall for resuts.
+func noopCFunction() {
+	C.noop_c_function()
+}