Fix license on rsaz_exp.h.

Imported from upstream's 4102d1a5034aea9cf3b070534bc97f5cee2f2296.

Change-Id: I6ddfa8ff88101e330e74958a3ae776f8758eb724
Reviewed-on: https://boringssl-review.googlesource.com/5484
Reviewed-by: Adam Langley <agl@google.com>