Remove unused #include <errno.h>

Found with iwyu.

rigs/flexradio/smartsdr.c

@@ -23,7 +23,6 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <math.h>
-#include <errno.h>
 
 #include "hamlib/rig.h"
 #include "parallel.h"

rigs/kit/dwt.c

@@ -505,7 +505,6 @@ static const char *dwtdll_get_info(RIG *rig)
 
 #elif defined(HAVE_LIBUSB) && (defined(HAVE_LIBUSB_H) || defined(HAVE_LIBUSB_1_0_LIBUSB_H))
 
-#include <errno.h>
 
 #ifdef HAVE_LIBUSB_H
 # include <libusb.h>

rigs/kit/elektor507.c

@@ -148,7 +148,6 @@ struct elektor507_extra_priv_data
 #elif defined(USE_LIBUSB)
 
 
-#include <errno.h>
 
 #ifdef HAVE_LIBUSB_H
 # include <libusb.h>

rigs/kit/fifisdr.c

@@ -39,7 +39,6 @@
  */
 #if defined(HAVE_LIBUSB) && (defined(HAVE_LIBUSB_H) || defined(HAVE_LIBUSB_1_0_LIBUSB_H))
 
-#include <errno.h>
 
 #ifdef HAVE_LIBUSB_H
 # include <libusb.h>

rigs/kit/funcube.c

@@ -37,7 +37,6 @@
  */
 #if defined(HAVE_LIBUSB) && (defined(HAVE_LIBUSB_H) || defined(HAVE_LIBUSB_1_0_LIBUSB_H))
 
-#include <errno.h>
 
 #ifdef HAVE_LIBUSB_H
 # include <libusb.h>

rigs/kit/si570avrusb.c

@@ -41,7 +41,6 @@
  */
 #if defined(HAVE_LIBUSB) && (defined(HAVE_LIBUSB_H) || defined(HAVE_LIBUSB_1_0_LIBUSB_H))
 
-#include <errno.h>
 
 #ifdef HAVE_LIBUSB_H
 # include <libusb.h>

src/sleep.c

@@ -36,7 +36,6 @@
  *
  */
 #include <unistd.h>
-#include <errno.h>
 #include <pthread.h>
 #include "hamlib/config.h"
 #include "sleep.h"