cmake : do not include ./src as public for libllama

[ggerganov]

• No longer include ./src as public path in libllama
• Move examples/llama-gbnf-validator -> tests/test-gbnf-validator
• Move examples/llama-quantize-stats -> tests/test-quantize-stats

With this change, examples no longer include files from ./src.

[slaren]

This should also be changed to PRIVATE, making it public may force applications to use C++17.

llama.cpp/src/CMakeLists.txt

Line 37 in eaf5a1e

target_compile_features (llama PUBLIC cxx_std_17) # don't bump