Update fftw config test.txt

[CI SKIP]

config/has_fftw3.cpp

@@ -7,10 +7,13 @@
 
 int main()
 {
+   fftwq_plan plan;  // early versions don't have this it seems.
+
    fftw_cleanup();
    fftwf_cleanup();
    fftwl_cleanup();
 
+
    return 0;
 }