x86, vdso, build: Don't rebuild 32-bit vdsos on every make

vdso32/vclock_gettime.o was confusing kbuild.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Stefani Seibold <stefani@seibold.net>
Link: http://lkml.kernel.org/r/d741449340642213744dd659471a35bb970a0c4c.1395789923.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/Makefile

@@ -136,7 +136,7 @@ override obj-dirs = $(dir $(obj)) $(obj)/vdso32/
 
 targets += vdso32/vdso32.lds
 targets += $(vdso32-images) $(vdso32-images:=.dbg)
-targets += vdso32/note.o $(vdso32.so-y:%=vdso32/%.o)
+targets += vdso32/note.o vdso32/vclock_gettime.o $(vdso32.so-y:%=vdso32/%.o)
 
 extra-y	+= $(vdso32-images)