Fix activation block in gemm.cl

- Replace VEC_SIZE with N0. VEC_SIZE was used in the old gemm kernel and
  not used anymore in the existing ones

Resolves COMPMID-5678

Change-Id: Ia770200b9d6e24c51c57347e4634fb8eadd10385
Signed-off-by: Gian Marco Iodice <gianmarco.iodice@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/8556
Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Reviewed-by: Gunes Bayir <gunes.bayir@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Benchmark: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
diff --git a/tests/validation/CL/GEMMMatrixMultiplyReshapedOnlyRhsMMUL.cpp b/tests/validation/CL/GEMMMatrixMultiplyReshapedOnlyRhsMMUL.cpp
index 7808be8..3b3cf85 100644
--- a/tests/validation/CL/GEMMMatrixMultiplyReshapedOnlyRhsMMUL.cpp
+++ b/tests/validation/CL/GEMMMatrixMultiplyReshapedOnlyRhsMMUL.cpp
@@ -81,6 +81,7 @@
 const auto act_values = framework::dataset::make("Activation",
 {
     ActivationLayerInfo(ActivationLayerInfo::ActivationFunction::RELU),
+    ActivationLayerInfo(ActivationLayerInfo::ActivationFunction::ELU),
 });
 
 /** M0 values to test - Precommit */