Fix more pedantic errors in pffft

pffft/pffft_double.h

@@ -186,10 +186,10 @@ extern "C" {
   void pffftd_zconvolve_no_accu(PFFFTD_Setup *setup, const double *dft_a, const double *dft_b, double*dft_ab, double scaling);
 
   /* return 4 or 1 wether support AVX instructions was enabled when building pffft-double.c */
-  int pffftd_simd_size();
+  int pffftd_simd_size(void);
 
   /* return string identifier of used architecture (AVX/..) */
-  const char * pffftd_simd_arch();
+  const char * pffftd_simd_arch(void);
 
   /* simple helper to get minimum possible fft size */
   int pffftd_min_fft_size(pffft_transform_t transform);