journal: don't export MESSAGE_ID() macro

If it is exported it would need to be prefixed, but since we need it
exclusively internally so far, simply move it to an internal header.

src/shared/log.h

@@ -135,3 +135,5 @@ bool log_on_console(void);
 
 const char *log_target_to_string(LogTarget target);
 LogTarget log_target_from_string(const char *s);
+
+#define MESSAGE_ID(x) "MESSAGE_ID=" SD_ID128_FORMAT_STR, SD_ID128_FORMAT_VAL(x)

src/systemd/sd-id128.h

@@ -65,7 +65,6 @@ static inline int sd_id128_equal(sd_id128_t a, sd_id128_t b) {
 }
 
 #define SD_ID128_NULL ((sd_id128_t) { .qwords = { 0, 0 }})
-#define MESSAGE_ID(x) "MESSAGE_ID=" SD_ID128_FORMAT_STR, SD_ID128_FORMAT_VAL(x)
 
 #ifdef __cplusplus
 }