pkg-config: depend on libiconv unconditionally

libiconv is already defined per-platform.  The actual libiconv library
won't be built on platforms like Linux where it doesn't need to be, so
there's no need to maintain a separate platform list here.

Fixes pkgsCross.x86_64-freebsd.pkg-config.

pkgs/development/tools/misc/pkg-config/default.nix

@@ -25,7 +25,7 @@ stdenv.mkDerivation rec {
     rm -f check/check-requires-private check/check-gtk check/missing
   '';
 
-  buildInputs = optional (stdenv.isCygwin || stdenv.isDarwin || stdenv.isSunOS) libiconv;
+  buildInputs = [ libiconv ];
 
   configureFlags = [ "--with-internal-glib" ]
     ++ optionals (stdenv.isSunOS) [ "--with-libiconv=gnu" "--with-system-library-path" "--with-system-include-path" "CFLAGS=-DENABLE_NLS" ]