os: fix build issue caused by fileno on *BSD

vlib/os/os.v

@@ -301,7 +301,11 @@ pub fn fileno(cfile voidptr) int {
 	$if windows {
 		return C._fileno(cfile)
 	} $else {
-		return C.fileno(cfile)
+		cfile_casted := &C.FILE(0) // FILE* cfile_casted = 0;
+		cfile_casted = cfile
+		// Required on FreeBSD/OpenBSD/NetBSD as stdio.h defines fileno(..) with a macro
+		// that performs a field access on its argument without casting from void*.
+		return C.fileno(cfile_casted)
 	}
 }