Include linux/sockios.h to fix musl build

Including <sys/ioctl.h> and <net/if.h> is not sufficient
to musl to find SIOCSIFNAME, so <linux/sockios.h> must be
included too.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

slcan_attach.c

@@ -52,6 +52,7 @@
 #include <net/if.h>
 #include <termios.h>
 #include <linux/tty.h>
+#include <linux/sockios.h>
 
 void print_usage(char *prg)
 {

slcand.c

@@ -38,6 +38,7 @@
 #include <net/if.h>
 #include <termios.h>
 #include <linux/tty.h>
+#include <linux/sockios.h>
 
 /* Change this to whatever your daemon is called */
 #define DAEMON_NAME "slcand"