Fixes clang build with SSE4 disabled

Author: Dmitry Vedenko

celt/cpu_support.h

@@ -62,9 +62,6 @@ int opus_select_arch(void);
 #else
 #define OPUS_ARCHMASK 0
 
-static OPUS_INLINE int opus_select_arch(void)
-{
-  return 0;
-}
+int opus_select_arch(void);
 #endif
 #endif