um/vdso: add .gitignore for a couple of targets

Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/vdso/.gitignore

@@ -0,0 +1,2 @@
+vdso-syms.lds
+vdso.lds