charset: make u16_strnlen accessible at runtime

commit 1fabfeef506c ("efi_loader: parameter check in GetNextVariableName()")
introduces a check using u16_strnlen(). This code is used on EFI
runtime variables as well, so unless we mark it as runtime, the kernel
will crash trying to access it.

Fixes: 1fabfeef506c ("efi_loader: parameter check in GetNextVariableName()")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>