Set supervisor as volatile as it is modified from an interrupt

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402335

src/timer/mint/SDL_systimer.c

@@ -53,7 +53,7 @@ void SDL_MintAudio_CheckFpu(void);
 
 /* The first ticks value of the application */
 static Uint32 start;
-static SDL_bool supervisor;
+static volatile SDL_bool supervisor;
 static int mint_present; /* can we use Syield() ? */
 
 void SDL_StartTicks(void)