package/libglib2: fix libiconv handling

package/libglib2: fix libiconv handling

In upstream commit e71ecc8771a4f13bc6046438ab0845944831b9a6 ("build:
Remove deprecated -Diconv option"), merged since glib 2.75.1, the
meson -Diconv option was removed.

In Buildroot, this means that the build of libglib2 has been broken
since commit 3f9622fe3d85b4d9f734b879abc1d485afea19b2, which bumped
libglib2 from 2.72.3 to 2.76.1 for configurations that have libiconv
enabled, causing this build failure:

../output-1/build/libglib2-2.76.1/meson.build:1:0: ERROR: Unknown options: "iconv"

iconv is now automatically detected by Meson machinery, and so the
option was considered as no longer being needed. This commit fixes
that by dropping the useless -Diconv=external.

Another related change done is remove the double addition of libiconv
into the <pkg>_DEPENDENCIES variable: libiconv can only be enabled
when BR2_ENABLE_LOCALE is disabled, and libglib2/Config.in selects
BR2_PACKAGE_LIBICONV when !BR2_ENABLE_LOCALE. So testing
BR2_ENABLE_LOCALE!=y and BR2_PACKAGE_LIBICONV=y is exactly the same
thing, causing libiconv to be added twice to the dependencies.

Fixes:

  http://autobuild.buildroot.net/results/d2da03f7558f3b6ee59c813bb64115702e52704c/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>