efi_loader: eliminate duplicate runtime section definitions

The following symbols are defined in two includes:

* __efi_runtime_start[]
* __efi_runtime_stop[]
* __efi_runtime_rel_start[]
* __efi_runtime_rel_stop[]

Eliminate the definitions in efi_loader.h.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>