hexagon: fix OpenCL not found error when building hexagon backend

[Russyyds]

Overview

Fix compile error for hexagon backend

Additional information

Fixes #22945

Add OpenCL libs when compile hexagon backend.

1. Download the OpenCL from Qualcomm website or KhronosGroup
2. Add the OpenCL dir to environ and run the command below

export ANDROID_NDK_ROOT=/home/tom/codes/android-ndk-r27d
export OpenCL_LIBRARY=/path/to/opencl-sdk/libs/Android/libOpenCL.so
export OpenCL_INCLUDE_DIR=/path/to/opencl-sdk/inc
export HEXAGON_SDK_ROOT=/path/to/Hexagon_SDK/6.4.0.2
export HEXAGON_TOOLS_ROOT=/path/to/Hexagon_SDK/6.4.0.2/tools/HEXAGON_Tools/19.0.04

mkdir -p build-snapdragon
cp docs/backend/snapdragon/CMakeUserPresets.json .
cmake --preset arm64-android-snapdragon-release -B build-snapdragon
cmake --build build-snapdragon

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: NO

[max-krasnyansky]

@Russyyds
This is not Hexagon related, it's more of a Snapdragon build related.
ie enabling CPU/GPU/NPU backends in one build as we recommend by default in the README.
If you use the docker toolchain (see the readme) everything is installed in the correct locations so there is no need to add additional env/cmake vars.