Move nsswitch.h into libc/dns/include.

Not a public header file except in NetBSD, where it came from. Even
OpenBSD doesn't have it.

Bug: http://b/28519060
Change-Id: Id5feec4a4e8010a9f4433e73b1bfa6f935d884b9
diff --git a/libc/include/nsswitch.h b/libc/dns/include/nsswitch.h
similarity index 100%
rename from libc/include/nsswitch.h
rename to libc/dns/include/nsswitch.h