efi: Export efi_query_variable_store() for efivars.ko

Fixes build with CONFIG_EFI_VARS=m which was broken after the commit
"x86, efivars: firmware bug workarounds should be in platform code".

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>