avoid a compiler warning, (unused static function)

src/frontend/com_fft.c

@@ -21,7 +21,9 @@ Author:   2008 Holger Vogt
 #include "ngspice/fftext.h"
 
 
+#ifndef GREEN
 static void fftext(double*, double*, long int, long int, int);
+#endif
 
 
 void
@@ -582,6 +584,8 @@ done:
 }
 
 
+#ifndef GREEN
+
 static void
 fftext(double *x, double *y, long int n, long int nn, int dir)
 {
@@ -668,3 +672,5 @@ fftext(double *x, double *y, long int n, long int nn, int dir)
         }
     }
 }
+
+#endif /* GREEN */