Fix buffer overflow in Backup.Creator

I noticed that while testing the upcoming GCC 12 with -D_FORTIFY_SOURCE=3:
here I have

$1 = 0x7ffff7f0f940 <Buffer.1.lto_priv.1> "Linux, kernel 5.16.14-1-default (#1 SMP PREEMPT Fri Mar 11 12:33:34 UTC 2022 (80acc65))"
(gdb) p (int)strlen(GetOS())
$3 = 87
so GetOS() returns 87 chars while:

include/gammu-backup.h: char Creator[80];

Fixes: #701

include/gammu-backup.h

@@ -218,7 +218,7 @@ typedef struct {
 	/**
 	 * Name of program which created backup
 	 */
-	char Creator[80];
+	char Creator[512];
 	/**
 	 * Timestamp of backup
 	 */