x86/vdso: Teach 'make clean' to remove generated vdso-image-*.c files

x86/vdso: Teach 'make clean' to remove generated vdso-image-*.c files

After 'make clean' the following files were left in arch/x86/vdso/:

  vdso-image-32-int80.c
  vdso-image-32-syscall.c
  vdso-image-32-sysenter.c

These file are generated during the build process and are present
in .gitignore, so remove them.

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/f85bb7ef6f8c6f6aa4bf422348018c84321454f8.1427482099.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>