Pull in syscall_table.S when building in separate object directory.

When building in separate object directory entry.o fails building because entry.S includes syscall_table.S but it's in the wrong directory. This simply adds the obj-dir to the include search path.

@@ -92,6 +92,7 @@
 
 $(obj)/vsyscall-int80.S: $(obj)/vsyscall-sigreturn.S
 
+EXTRA_AFLAGS	+= -I$(obj)
 $(obj)/entry.o: $(src)/entry.S $(src)/syscall_table.S
 
 obj-y	+= $(c-obj-y) $(s-obj-y)