change the check for a Windows partition in chainloader_func, to fix the problem that GRUB cannot be compiled, if FAT support is disabled.

stage2/shared.h

@@ -175,6 +175,7 @@ extern char *grub_scratch_mem;
 #define BOOTSEC_SIGNATURE		0xAA55
 #define BOOTSEC_BPB_OFFSET		0x3
 #define BOOTSEC_BPB_LENGTH		0x3B
+#define BOOTSEC_BPB_SYSTEM_ID		0x3
 #define BOOTSEC_BPB_HIDDEN_SECTORS	0x1C
 #define BOOTSEC_PART_OFFSET		0x1BE
 #define BOOTSEC_PART_LENGTH		0x40