package/libvirt: fix dependencies in Config.in

Drop spurious "depends on WCHAR" added with commit
ccfc90e1 and adds missing wchar comment

While at it, drop BR2_USE_MMU from comment as it is already added by
BR2_PACKAGE_LIBVIRT_ARCH_SUPPORTS and fix indentation before
(BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_11 || !BR2_aarch64))

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>