sdl12-compat: always define DRMP3_API -- just in case.

SDL Commits
1 
From 7c391e4783fae5b58a5e18ae8116148c144e77b0 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Tue, 15 Jun 2021 12:29:20 +0300
Subject: [PATCH] always define DRMP3_API -- just in case.

---
 src/SDL12_compat.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/SDL12_compat.c b/src/SDL12_compat.c
index e17c1b5..f8c6f10 100644
--- a/src/SDL12_compat.c
+++ b/src/SDL12_compat.c
@@ -5953,6 +5953,8 @@ SDL_LoadWAV_RW(SDL12_RWops *rwops12, int freerwops12,
 #define DRMP3_API __attribute__((visibility("hidden")))
 #elif defined(__APPLE__)
 #define DRMP3_API __private_extern__
+#else
+#define DRMP3_API /* just in case */
 #endif
 #define DR_MP3_NO_STDIO 1
 #define DR_MP3_NO_S16 1