Bugfix: Only define have_libusb if building with libusb support

miner.c

@@ -191,10 +191,9 @@ bool use_curses = true;
 #else
 bool use_curses;
 #endif
-#ifndef HAVE_LIBUSB
-const
-#endif
+#ifdef HAVE_LIBUSB
 bool have_libusb;
+#endif
 static bool opt_submit_stale = true;
 static int opt_shares;
 static int opt_submit_threads = 0x40;

miner.h

@@ -903,7 +903,9 @@ extern bool opt_fail_only;
 extern bool opt_autofan;
 extern bool opt_autoengine;
 extern bool use_curses;
+#ifdef HAVE_LIBUSB
 extern bool have_libusb;
+#endif
 extern int httpsrv_port;
 extern int stratumsrv_port;
 extern char *opt_api_allow;