[build] Add yet another potential location for isolinux.bin

Reported-by: Martin Sofaru <ipxe@fhloston.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/Makefile

 	/usr/share/syslinux/isolinux.bin \
 	/usr/share/syslinux/bios/isolinux.bin \
 	/usr/local/share/syslinux/isolinux.bin \
-	/usr/local/share/syslinux/bios/isolinux.bin
+	/usr/local/share/syslinux/bios/isolinux.bin \
+	/usr/lib/ISOLINUX/isolinux.bin
 ISOLINUX_BIN	= $(firstword $(wildcard $(ISOLINUX_BIN_LIST)))
 
 # i386-specific directories containing source files