random:drbg: Fix the behavior for child process.

* random/random-drbg.c (_gcry_rngdrbg_randomize): Update change of PID
detection.

In a child process, it calls to drbg_reseed again and again, without
this change.

(cherry picked from commit df7879a86b1de8eaf2d784687155c4274574b120)

Note that this RNG is only used in FIPS mode.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>