Fix tdemultimedia FTBFS when gcc hidden visibility is enabled

(cherry picked from commit 7395916e07)

flow/fft.h

@@ -9,6 +9,7 @@
  * compatible to avoid everybody to reimplement/duplicate their own fft.
  */
 
+#include "arts_export.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -23,8 +24,8 @@ void arts_fft_float (
     float    *RealIn,              /* array of input's real samples */
     float    *ImaginaryIn,         /* array of input's imag samples */
     float    *RealOut,             /* array of output's reals */
-    float    *ImaginaryOut );      /* array of output's imaginaries */
-
+    float    *ImaginaryOut )      /* array of output's imaginaries */
+    ARTS_EXPORT;
 
 #ifdef __cplusplus
 }