Fix c++ identity crisis in external_metro

The program is C++ but includes C headers and uses extern "C"
clauses around the ExternalMetro class.

tests/external_metro.cpp

@@ -17,7 +17,7 @@
 */
 
 #include "external_metro.h"
-#include <stdio.h>
+#include <cstdio>
 
 typedef jack_default_audio_sample_t sample_t;
 

tests/external_metro.h

@@ -21,18 +21,13 @@
 #ifndef __external_metro__
 #define __external_metro__
 
-#ifdef __cplusplus
-extern "C"
-{
-#endif
-
-#include <stdlib.h>
-#include <stdio.h>
-#include <errno.h>
+#include <cstdlib>
+#include <cstdio>
+#include <cerrno>
 #include <unistd.h>
-#include <math.h>
+#include <cmath>
 #include <getopt.h>
-#include <string.h>
+#include <cstring>
 #include <jack/jack.h>
 #include <jack/transport.h>
 
@@ -63,8 +58,4 @@ extern "C"
         static void shutdown (void* arg);
     };
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif