sdl2-compat: add a forward declaration of SDL_NumJoysticks() to fix build.

SDL Commits
#1 
From bb66ed056b8a803677b23082438677a5676e3f22 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Tue, 7 Feb 2023 13:32:00 +0300
Subject: [PATCH] add a forward declaration of SDL_NumJoysticks() to fix build.

---
 src/sdl2_compat.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/sdl2_compat.c b/src/sdl2_compat.c
index 23963d4..98370a3 100644
--- a/src/sdl2_compat.c
+++ b/src/sdl2_compat.c
@@ -336,6 +336,10 @@ SDL2Compat_GetHintBoolean(const char *name, SDL_bool default_value)
 }
 
 
+/* Forward declarations */
+DECLSPEC int SDLCALL SDL_NumJoysticks(void);
+
+
 /* if you change this, update also SDL2Compat_ApplyQuirks() */
 static const char *
 SDL2_to_SDL3_hint(const char *name)