os: eliminate warning for v -cross -o vc/v.c cmd/v on linux (part 2, see also 037a3b4)

Author: spytheman

vlib/os/os.c.v

@@ -527,7 +527,7 @@ pub fn get_raw_line() string {
 		unsafe {
 			initial_size := 256 * wide_char_size
 			mut buf := malloc_noscan(initial_size)
-			defer { buf.free() }
+			defer { unsafe { buf.free() } }
 			mut capacity := initial_size
 			mut offset := 0