x86/um: thin archives build fix

The linker does not like vdso-syms.lds in input archive files.
Make it an extra-y instead.

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: user-mode-linux-devel@lists.sourceforge.net
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/x86/um/vdso/Makefile

@@ -53,7 +53,7 @@ CFLAGS_REMOVE_vdso-note.o = -pg -fprofile-arcs -ftest-coverage
 CFLAGS_REMOVE_um_vdso.o = -pg -fprofile-arcs -ftest-coverage
 
 targets += vdso-syms.lds
-obj-$(VDSO64-y)			+= vdso-syms.lds
+extra-$(VDSO64-y)			+= vdso-syms.lds
 
 #
 # Match symbols in the DSO that look like VDSO*; produce a file of constants.