(!1688) os: unexport internal logging functions

Lots of logging functions, especially init and teardown aren't called
by any drivers/modules, so no need to keep them exported.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

test/signal-logging.c

@@ -30,6 +30,7 @@
 #include <unistd.h>
 
 #include "os/fmt.h"
+#include "os/log_priv.h"
 
 #include "assert.h"
 #include "misc.h"