[prefix] Fix missing underscore in libprefix

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

 	.section ".prefix.data", "aw", @progbits
 progress_\@:
 	.asciz	"\message"
-	.size	progress_\@, . - progress\@
+	.size	progress_\@, . - progress_\@
 	.previous
 	.endm
 #endif