common: Avoid warning about implicit declaration of gnupg_fd_valid.

* common/logging.c: Add #include "sysutils.h".

Without this, we see:

logging.c:573:9: warning: implicit declaration of function \

‘gnupg_fd_valid’ [-Wimplicit-function-declaration]
 if (! gnupg_fd_valid (fd))
       ^~~~~~~~~~~~~~

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>