Fix test failure on windows

This patch ensures that the multiple extern definitions of `free` in the
run-pass tests have the same declaration, working around #7352.

src/test/run-pass/extern-pub.rs

@@ -1,7 +1,5 @@
-use std::libc;
-
 extern {
-    pub unsafe fn free(p: *libc::c_void);
+    pub unsafe fn free(p: *u8);
 }
 
 pub fn main() {