efibootmgr: depends on wchar

efibootmgr: depends on wchar

After commit 3ae07b4746 recently, efibootmgr now selects
BR2_PACKAGE_GETTEXT if the toolchain requires it.
gettext depends on wchar, so this dependency should be propagated as
well.
menuconfig currently complains loudly if you select efibootmgr, with an
error such as:

  warning: (... && BR2_PACKAGE_EFIBOOTMGR ... && ) selects
  BR2_PACKAGE_GETTEXT which has unmet direct dependencies
  (BR2_USE_WCHAR)

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>