libiconv: don't use libiconvReal on NetBSD

It's perhaps debatable whether we want to be building iconv as part of
libc on BSD, I suppose, but while that's the current situation there's
no point pulling in libiconvReal as well.

pkgs/top-level/all-packages.nix

@@ -21110,9 +21110,10 @@ with pkgs;
 
   libgsf = callPackage ../development/libraries/libgsf { };
 
-  # GNU libc provides libiconv so systems with glibc don't need to build
-  # libiconv separately. Additionally, Apple forked/repackaged libiconv so we
-  # use that instead of the vanilla version on that OS.
+  # GNU libc provides libiconv so systems with glibc don't need to
+  # build libiconv separately. Additionally, Apple forked/repackaged
+  # libiconv so we use that instead of the vanilla version on that OS,
+  # and BSDs include libiconv in libc.
   #
   # We also provide `libiconvReal`, which will always be a standalone libiconv,
   # just in case you want it regardless of platform.
@@ -21123,6 +21124,8 @@ with pkgs;
         else stdenv.cc.libc)
     else if stdenv.hostPlatform.isDarwin
       then darwin.libiconv
+    else if stdenv.hostPlatform.isNetBSD
+      then null
     else libiconvReal;
 
   glibcIconv = libc: let