misc/cgo/testcarchive: add missing exit(0) in main7.c

Fixes #35327

Change-Id: I3726bfad24851a0bef8891014f7c5a7c48352307
Reviewed-on: https://go-review.googlesource.com/c/go/+/205077
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testcarchive/testdata/main7.c

@@ -14,4 +14,5 @@ int main() {
 	for (i = 0; i < 100000; i++) {
 		GoFunction7();
 	}
+	return 0;
 }