Core: Remove disallowed register.

Core/Util/AudioFormat.h

@@ -49,7 +49,7 @@ static inline s16 clamp_s16(int i) {
 
 static inline s16 ApplySampleVolume(s16 sample, int vol) {
 #if PPSSPP_ARCH(ARM) && !defined(_MSC_VER)
-	register int r;
+	int r;
 	asm volatile("smulwb %0, %1, %2\n\t" \
 	             "ssat %0, #16, %0" \
 	             : "=r"(r) : "r"(vol), "r"(sample));