fix linker error bug with fftw3 and -DFFT_SINGLE

src/main.cpp

@@ -70,6 +70,10 @@ int main(int argc, char **argv)
 #ifdef FFT_FFTW3
   // tell fftw3 to delete its global memory pool
   // and thus avoid bogus valgrind memory leak reports
+#ifdef FFT_SINGLE
+  fftwf_cleanup();
+#else
   fftw_cleanup();
 #endif
+#endif
 }