snd_sb.c: Fix compilation error

Variable declarations, unlike statements, aren't allowed after
labels (including case labels) according to the C standard,
so insert a semicolon to make an empty statement that satisfies
the requirement.

Oddly, MSYS2/MinGW-w64 GCC, unlike other compilers, including GCC
on other systems, seemed to accept them without errors...

src/sound/snd_sb.c

@@ -1292,6 +1292,7 @@ sb_ct1745_mixer_read(uint16_t addr, void *priv)
                 break;
 
             case 0x82:
+                ; /* Empty statement to make compilers happy about the following variable declaration. */
                 /* The Interrupt status register, addressed as register 82h on the Mixer register map,
                    is used by the ISR to determine whether the interrupt is meant for it or for some
                    other ISR, in which case it should chain to the previous routine. */