random: Allow building rndjent.c with stats collecting enabled.

* random/rndjent.c: Change license to the one used by jitterentropy.h.
(jent_init_statistic): New.
(jent_bit_count): New.
(jent_statistic_copy_stat): new.
(jent_calc_statistic): New.

New code taken from Stephan's jitterentropy-stat.c. This does now
build with CONFIG_CRYPTO_CPU_JITTERENTROPY_STAT defined; not sure
whether this is already useful. Changed the license due to the new
code.

• Signed-off-by: Werner Koch <wk@gnupg.org>