Add IWYU export pragmas to top-level headers.

unsupported/Eigen/FFT

@@ -79,6 +79,8 @@
 
 #include "../../Eigen/src/Core/util/DisableStupidWarnings.h"
 
+// IWYU pragma: begin_exports
+
 #ifdef EIGEN_FFTW_DEFAULT
 // FFTW: faster, GPL -- incompatible with Eigen in LGPL form, bigger code size
 #  include <fftw3.h>
@@ -110,6 +112,8 @@
   }
 #endif
 
+// IWYU pragma: end_exports
+
 namespace Eigen {