Avoid implicit int and implicit function declarations.

A mostly equivalent patch has been posted upstream:

<https://sourceforge.net/p/setserial/discussion/7060/thread/95d874c12c/?limit=25#1643>

diff --git a/setserial.c b/setserial.c
index f184e98db07ceb59..d181327fc568ede4 100644
--- a/setserial.c
+++ b/setserial.c
@@ -15,6 +15,9 @@
 #include <termios.h>
 #include <string.h>
 #include <errno.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+#include <stdlib.h>
 
 #ifdef HAVE_ASM_IOCTLS_H
 #include <asm/ioctls.h>
@@ -714,6 +717,7 @@ fprintf(stderr, "\t* port\t\tset the I/O port\n");
 	exit(1);
 }
 
+int
 main(int argc, char **argv)
 {
 	int	get_flag = 0, wild_intr_flag = 0;