jitterentropy: Include <fcntl.h> and <limits.h>

* random/jitterentropy-base-user.h: Include <fcntl.h> for O_RDONLY
* random/jitterentropy-base-user.h: Include <limits.h> for LONG_MAX

Cherry-picked from LIBGCRYPT-1.10-BRANCH branch of:
ffaef0be613121d3ee37867d82932a7a30c2bc6d

Fixes the build with musl libc.

• Signed-off-by: Heiko Becker <heirecka@exherbo.org>