MAINTAINERS: add entry for crypto library

I am volunteering to maintain the kernel's crypto library code.

[ And Jason and Ard piped up too  - Linus ]

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>