package/guile: Fix build failure due to missing libiconv

When building package guile with BR2_ENABLE_LOCALE=n
we get the following error:

```
In file included from filesys.c:95:
ports-internal.h:26:10: fatal error: iconv.h: No such file or directory
   26 | #include <iconv.h>
      |          ^~~~~~~~~
compilation terminated.
```

So let's fix it by selecting BR2_PACKAGE_LIBICONV and
adding libiconv package as dependency.

Signed-off-by: Pedro Aguilar <paguilar@paguilar.org>
Reviewed-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Tested-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/guile/Config.in

@@ -9,6 +9,7 @@ config BR2_PACKAGE_GUILE
 	select BR2_PACKAGE_LIBFFI
 	select BR2_PACKAGE_GMP
 	select BR2_PACKAGE_BDWGC
+	select BR2_PACKAGE_LIBICONV if !BR2_ENABLE_LOCALE
 	help
 	  Guile is an interpreter and compiler for the Scheme
 	  programming language, a clean and elegant dialect of Lisp.

package/guile/guile.mk

@@ -18,6 +18,10 @@ GUILE_CPE_ID_VENDOR = gnu
 GUILE_DEPENDENCIES = host-guile libunistring libffi gmp bdwgc host-pkgconf
 HOST_GUILE_DEPENDENCIES = host-libunistring host-libffi host-gmp host-bdwgc host-flex host-pkgconf host-gettext
 
+ifeq ($(BR2_ENABLE_LOCALE),)
+GUILE_DEPENDENCIES += libiconv
+endif
+
 # The HAVE_GC* CFLAGS specify that we will use internal callbacks
 # instead of the ones provided by
 # bdwgc. Eg. HAVE_GC_SET_FINALIZER_NOTIFIER specifies that we won't