Fix mismatch between declaration and forward declaration

KernelArgumentBlock is defined as a class in KernelArgumentBlock.h, but
forward declarations refer to it as a struct.

While this is essentially the same, the mismatch causes a compiler
warning in clang (and may cause warnings in future versions of gcc) in
code that is supposed to be compiled with -Werror.

Change-Id: I4ba49d364c44d0a42c276aff3a8098300dbdcdf0
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>