[build] Include ipxe.pxe in default build

Signed-off-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 # helpfully suggestive message
 #
 ALL		:= bin/blib.a bin/ipxe.dsk bin/ipxe.lkrn bin/ipxe.iso \
-		   bin/ipxe.usb bin/undionly.kpxe bin/rtl8139.rom
+		   bin/ipxe.usb bin/ipxe.pxe bin/undionly.kpxe bin/rtl8139.rom
 all : $(ALL)
 	@$(ECHO) '==========================================================='
 	@$(ECHO)