Fixed macro redefinition in Half.hpp
Fixed macro redefinition in Half.hpp

* Fixed the compiler warnings about macro redefinitions when building the driver

* Resolves MLCE-622

Change-Id: I7231f9772dc0f38191853ff378670d45648ef7f6
Signed-off-by: Pablo Tello <3fe6eac348616d7069b899a4deab22c7c8709a13@arm.com>

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn