Fix CMakeLists.txt to handle deprecated gpu Warnings

[jugal-sheth]

Added -Wno-deprecated-gpu-targets instead of -Wno-deprecated

[jugal-sheth]

This new change updates the CMake configuration to apply -Wno-deprecated-gpu-targets only when GGML_CUDA is enabled.
Replaced the global CMAKE_CXX_FLAGS override with a conditional:

if (GGML_CUDA)
    add_compile_options(-Wno-deprecated-gpu-targets)
endif()

Should prevent non-CUDA platforms from throwing warnings like:

warning: unknown warning option '-Wno-deprecated-gpu-targets'; did you mean '-Wno-deprecated-pragma'?

Apologies for missing this earlier I didn't catch that applying this flag unconditionally would trigger issues on non-CUDA systems. Thanks for pointing it out. This change is intended to make sure that warning is handled without polluting global flags