cmd/go: add declaration to cgo_lto_issue43830 test

This permits the test to work in C99 mode.

For #43830

Change-Id: Ide54bd62239cfe602e2664300f04e472df5daf43
Reviewed-on: https://go-review.googlesource.com/c/go/+/324009
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: ianlancetaylor

src/cmd/go/testdata/script/cgo_lto_issue43830.txt

@@ -15,6 +15,7 @@ go build main.go add.go
 package main
 
 /*
+extern int myadd(int, int);
 int c_add(int a, int b) {
 	return myadd(a, b);
 }