[build] Avoid rebuilding keymap files automatically

The keymap files, though autogenerated, are checked in to version
control and should be considered as source files.  They should never
be automatically rebuilt.

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

src/Makefile.housekeeping

 # Keyboard maps
 #
 
-hci/keymap/keymap_%.c : $(GENKEYMAP)
+hci/keymap/keymap_%.c :
 	$(Q)$(PERL) $(GENKEYMAP) $* > $@
 
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