ReSID: include math.h before trying to define M_FOO constants

This fixes a warning later, when the platform math.h gets included and
unconditionally tries to (re-)define the M_FOO constants.

src/sound/resid-fp/siddefs-fp.h

@@ -20,6 +20,8 @@
 #ifndef __SIDDEFS_FP_H__
 #define __SIDDEFS_FP_H__
 
+#include <math.h>
+
 #ifndef M_PI
 #define M_PI    3.14159265358979323846
 #define M_PI_f  3.14159265358979323846f