Fix the following build warnings by adding a header for
the definitions shared between jitterentropy.c and
jitterentropy-kcapi.c. Fixes the following:

crypto/jitterentropy.c:445:5: warning: symbol 'jent_read_entropy' was not declared. Should it be static?
crypto/jitterentropy.c:475:18: warning: symbol 'jent_entropy_collector_alloc' was not declared. Should it be static?
crypto/jitterentropy.c:509:6: warning: symbol 'jent_entropy_collector_free' was not declared. Should it be static?
crypto/jitterentropy.c:516:5: warning: symbol 'jent_entropy_init' was not declared. Should it be static?
crypto/jitterentropy-kcapi.c:59:6: warning: symbol 'jent_zalloc' was not declared. Should it be static?
crypto/jitterentropy-kcapi.c:64:6: warning: symbol 'jent_zfree' was not declared. Should it be static?
crypto/jitterentropy-kcapi.c:69:5: warning: symbol 'jent_fips_enabled' was not declared. Should it be static?
crypto/jitterentropy-kcapi.c:74:6: warning: symbol 'jent_pan...