Be sure to add "void" to all function prototypes that take no args.

Omitting "void" leads to a K&R style declaration which was not intended.
diff --git a/src/boot.c b/src/boot.c
index c427c00..151b0d3 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -22,7 +22,7 @@
  ****************************************************************/
 
 void
-boot_setup()
+boot_setup(void)
 {
     if (! CONFIG_BOOT)
         return;
@@ -222,7 +222,7 @@
 
 // Show IPL option menu.
 static void
-interactive_bootmenu()
+interactive_bootmenu(void)
 {
     if (! CONFIG_BOOTMENU || ! qemu_cfg_show_boot_menu())
         return;
@@ -309,7 +309,7 @@
 
 // Prepare for boot - show menu and run bcvs.
 void
-boot_prep()
+boot_prep(void)
 {
     if (! CONFIG_BOOT)
         return;
@@ -487,7 +487,7 @@
 
 // Boot Failure recovery: try the next device.
 void VISIBLE32FLAT
-handle_18()
+handle_18(void)
 {
     debug_serial_setup();
     debug_enter(NULL, DEBUG_HDL_18);
@@ -499,7 +499,7 @@
 
 // INT 19h Boot Load Service Entry Point
 void VISIBLE32FLAT
-handle_19()
+handle_19(void)
 {
     debug_serial_setup();
     debug_enter(NULL, DEBUG_HDL_19);