Add void to signatures of argumentless functions

This fixes pedantic compiler warnings of the form:

    pffft.h:197:22: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
      197 |   int pffft_simd_size();
          |                      ^
          |                       void

Author: thirtythreeforty

pffft.h

@@ -194,10 +194,10 @@ extern "C" {
   void pffft_zconvolve_no_accu(PFFFT_Setup *setup, const float *dft_a, const float *dft_b, float *dft_ab, float scaling);
 
   /* return 4 or 1 wether support SSE/NEON/Altivec instructions was enabled when building pffft.c */
-  int pffft_simd_size();
+  int pffft_simd_size(void);
 
   /* return string identifier of used architecture (SSE/NEON/Altivec/..) */
-  const char * pffft_simd_arch();
+  const char * pffft_simd_arch(void);
 
 
   /* following functions are identical to the pffftd_ functions */

pffft_priv_impl.h

@@ -69,7 +69,7 @@
 #endif
 
 
-int FUNC_SIMD_SIZE() { return SIMD_SZ; }
+int FUNC_SIMD_SIZE(void) { return SIMD_SZ; }
 
 int FUNC_MIN_FFT_SIZE(pffft_transform_t transform) {
   /* unfortunately, the fft size must be a multiple of 16 for complex FFTs
@@ -109,7 +109,7 @@ int FUNC_NEAREST_SIZE(int N, pffft_transform_t cplx, int higher) {
       return N;
 }
 
-const char * FUNC_SIMD_ARCH() { return VARCH; }
+const char * FUNC_SIMD_ARCH(void) { return VARCH; }
 
 
 /*
@@ -1819,7 +1819,7 @@ void FUNC_TRANSFORM_ORDERED(SETUP_STRUCT *setup, const float *input, float *outp
 #define assertv4(v,f0,f1,f2,f3) assert(v.f[0] == (f0) && v.f[1] == (f1) && v.f[2] == (f2) && v.f[3] == (f3))
 
 /* detect bugs with the vector support macros */
-void FUNC_VALIDATE_SIMD_A() {
+void FUNC_VALIDATE_SIMD_A(void) {
   float f[16] = { 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 };
   v4sf_union a0, a1, a2, a3, t, u; 
   memcpy(a0.f, f, 4*sizeof(float));