Fix copy-pasta in header.

Increase scope of SIREN_DSP_SLOW to entire file.

src/dsp_slow.cpp

@@ -31,14 +31,14 @@
 
 #include "../include/dsp.h"
 
+#ifdef SIREN_DSP_SLOW
+
 // _USE_MATH_DEFINES must be defined to get M_PI in Windows
 #define _USE_MATH_DEFINES
 #include <math.h>
 
 #include <assert.h>
 
-#ifdef SIREN_DSP_SLOW
-
 namespace siren {
 namespace dsp {