cross-build: Define crypt_data in unistd.h for libcrypt

This is where it's defined in the base system, so is where libcrypt
expects it to exist when being built, and will be needed when being
bootstrapped in a subsequent commit.

MFC after: 1 week

(cherry picked from commit 8ceba27a5d51e2fcc0ea547d5051bd859d15233c)