ia64: remove unneeded extra-y in Makefile.gate

All the files listed in "extra-y" are generated according to the
dependency.  They are still needed in "targets" to include .*.cmd
for incremental building.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/ia64/kernel/Makefile.gate

 # The gate DSO image is built using a special linker script.
 
-targets += gate.so gate-syms.o
-
-extra-y += gate.so gate-syms.o gate.lds gate.o
+targets += gate.so gate-syms.o gate.lds gate.o
 
 CPPFLAGS_gate.lds := -P -C -U$(ARCH)