Remove HEADER_BSS_FILE_C module include guard

This include guard inside an object file comes as a surprise and
serves no purpose anymore. It seems like this object file was
included by crypto/threads/mttest.c at some time, but the include
directive was removed in commit bb8abd67

.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9365)