tests/shell: Init xtimer when stdio_rtt is used

tests/shell/main.c

@@ -24,6 +24,10 @@
 #include "shell_commands.h"
 #include "shell.h"
 
+#if MODULE_STDIO_RTT
+#include "xtimer.h"
+#endif
+
 static int print_teststart(int argc, char **argv)
 {
     (void) argc;
@@ -64,6 +68,9 @@ int main(void)
 
     printf("test_shell.\n");
 
+#if MODULE_STDIO_RTT
+    xtimer_init();
+#endif
 
     /* define buffer to be used by the shell */
     char line_buf[SHELL_DEFAULT_BUFSIZE];