OpenCL benchmarks now respect SECP256K1_BUILD_BENCH on main

[shrec]

What

• Fixed issue opencl: benchmark targets built unconditionally, ignoring SECP256K1_BUILD_BENCH #193 by guarding opencl_benchmark and opencl_bip352_benchmark behind SECP256K1_BUILD_BENCH in opencl/CMakeLists.txt.
• This brings OpenCL in line with the CUDA side and stops benchmark sources from being compiled when the parent project explicitly disables benchmarks.
• It removes the Windows/OpenCL CI failure mode where <CL/cl.h> from benchmark-only sources could still break a non-benchmark build.

Numbers

• Verified configure: SECP256K1_BUILD_BENCH=OFF, SECP256K1_BUILD_OPENCL=ON
• Result: target list no longer includes opencl_benchmark or opencl_bip352_benchmark
• Commit: 726e31bf

How to get it

• Branch: main
• Commit: 726e31bf
• A fresh CI run was triggered automatically on push.