[romprefix] Add missing addr32 prefix

Reported-by: Jedrzej Kalinowski <kalinoj1@iem.pw.edu.pl>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/romprefix.S

@@ -489,7 +489,7 @@ get_pmm_image_source:
 	xorw	%ax, %ax
 	movw	%ax, %es
 	movl	build_id, %eax
-	cmpl	%es:build_id(%esi), %eax
+	addr32 cmpl %es:build_id(%esi), %eax
 	je	1f
 	stc
 1:	popw	%es