Add assert.h to threads_pthread.c for NonStop thread compiles.
Add assert.h to threads_pthread.c for NonStop thread compiles.

Fixes: #15809

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15812)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl