package: add fftw library

fftw is a library for fast fourier transforms.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
diff --git a/package/Config.in b/package/Config.in
index a849266..8b9c816 100644
--- a/package/Config.in
+++ b/package/Config.in
@@ -338,6 +338,7 @@
 endmenu
 
 menu "Other"
+source "package/fftw/Config.in"
 source "package/libargtable2/Config.in"
 source "package/argp-standalone/Config.in"
 source "package/libatomic_ops/Config.in"