bsod: do not use STRLEN

The compiler optimizes strlen away, so we can use the simplest form that is
type safe and more natural. STRLEN is only for array initialization.

src/journal/bsod.c

@@ -103,7 +103,7 @@ static int acquire_first_emergency_log_message(char **ret) {
         if (r < 0)
                 return log_error_errno(r, "Failed to read journal message: %m");
 
-        message = memdup_suffix0((const char*)d + STRLEN("MESSAGE="), l - STRLEN("MESSAGE="));
+        message = memdup_suffix0((const char*)d + strlen("MESSAGE="), l - strlen("MESSAGE="));
         if (!message)
                 return log_oom();