Fix -Wstrict-prototypes warnings

e.g.
SDL3/src/hidapi/libusb/hid.c:636:31: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
  636 | static void start_event_thread()

src/hidapi/libusb/hid.c

@@ -633,7 +633,7 @@ static void *event_thread(void *param)
 	return NULL;
 }
 
-static void start_event_thread()
+static void start_event_thread(void)
 {
 	if (libusb_has_capability(LIBUSB_CAP_HAS_HOTPLUG)) {
 		int res = libusb_hotplug_register_callback(usb_context, LIBUSB_HOTPLUG_EVENT_DEVICE_ARRIVED | LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT, 0, LIBUSB_HOTPLUG_MATCH_ANY, LIBUSB_HOTPLUG_MATCH_ANY, LIBUSB_HOTPLUG_MATCH_ANY, hotplug_callback, NULL, &hotplug_callback_handle);
@@ -645,7 +645,7 @@ static void start_event_thread()
 	hidapi_thread_create(&event_thread_state, event_thread, NULL);
 }
 
-static void stop_event_thread()
+static void stop_event_thread(void)
 {
 	shutdown_event_thread = 1;
 	libusb_interrupt_event_handler(usb_context);

src/notification/cocoa/SDL_cocoanotification.m

@@ -88,7 +88,7 @@ API_AVAILABLE(macos(10.14))
 static UNUserNotificationCenter *center;
 static SDLNotificationDelegate *delegate;
 
-static bool ShouldEnableNotifications()
+static bool ShouldEnableNotifications(void)
 {
 #if defined(SDL_PLATFORM_MACOS)
     /* Notifications outside of an app bundle are unsupported, and will crash with an
@@ -354,7 +354,7 @@ bool SDL_RemoveNotification(SDL_NotificationID notification)
 }
 #endif
 
-void SDL_CleanupNotifications()
+void SDL_CleanupNotifications(void)
 {
     // TODO: Anything to do here?
 }

src/video/cocoa/SDL_cocoamouse.h

@@ -26,7 +26,7 @@
 #include "SDL_cocoavideo.h"
 
 extern bool Cocoa_InitMouse(SDL_VideoDevice *_this);
-extern NSWindow *Cocoa_GetMouseFocus();
+extern NSWindow *Cocoa_GetMouseFocus(void);
 extern void Cocoa_HandleMouseEvent(SDL_VideoDevice *_this, NSEvent *event);
 extern void Cocoa_HandleMouseWheel(SDL_Window *window, NSEvent *event);
 extern void Cocoa_HandleMouseWarp(CGFloat x, CGFloat y);

src/video/cocoa/SDL_cocoamouse.m

@@ -768,7 +768,7 @@ static void Cocoa_HandleTitleButtonEvent(SDL_VideoDevice *_this, NSEvent *event)
 
 static NSWindow *Cocoa_MouseFocus;
 
-NSWindow *Cocoa_GetMouseFocus()
+NSWindow *Cocoa_GetMouseFocus(void)
 {
     return Cocoa_MouseFocus;
 }

src/video/cocoa/SDL_cocoawindow.m

@@ -2241,7 +2241,7 @@ static void Cocoa_SendMouseButtonClicks(SDL_Mouse *mouse, NSEvent *theEvent, SDL
 }
 @end
 
-static void Cocoa_UpdateMouseFocus()
+static void Cocoa_UpdateMouseFocus(void)
 {
     const NSPoint mouseLocation = [NSEvent mouseLocation];