Make DEBUG_printf() a proper function, implementation is port-dependent.

In particular, unix outputs to stderr, to allow to run testsuite against
micropython built with debug output (by redirecting stderr to /dev/null).
diff --git a/py/misc.h b/py/misc.h
index 065b070..989650c 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -117,4 +117,7 @@
 void vstr_vprintf(vstr_t *vstr, const char *fmt, va_list ap);
 #endif
 
+// Debugging helpers
+int DEBUG_printf(const char *fmt, ...);
+
 #endif // _INCLUDED_MINILIB_H