Add support for building with mingw

[jeberger]

There are two issues when attempting to build with mingw:

• llama_cpp_sys/build.rs adds MSVC compilation flags, which gcc doesn't understand,
• and it should add -DLLAMA_WIN_VER=0x602 (see Help: when "cmake --build . --config Release" , error: unknown type name 'FINDEX_INFO_LEVELS' ggml-org/llama.cpp#5158 (comment))

This patch fixes both issues: mingw.patch