[log] Flush after each message when logging to stderr

When logging to stderr from usb-moded systemd service (e.g. due to slightly
different logging format) block buffering is used and causes the messages
to appear out of sync vs other processes.

Flush stderr after each diagnostic message.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

src/usb_moded-log.c

@@ -279,6 +279,7 @@ void log_emit_va(const char *file, const char *func, int line, int lev, const ch
                         lineinfo, timeinfo, levelinfo, msg);
 #endif
             }
+            fflush(stderr);
             break;
 
         default: