build : move _WIN32_WINNT definition to headers (#17736)

Previously, cmake was forcing `_WIN32_WINNT=0x0A00` for MinGW builds,
This caused "macro redefined" warnings with toolchains that define the version.

This also removes the `GGML_WIN_VER` variable as it is no longer needed.

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

CMakeLists.txt

@@ -199,11 +199,6 @@ if (NOT TARGET ggml AND NOT LLAMA_USE_SYSTEM_GGML)
     # ... otherwise assume ggml is added by a parent CMakeLists.txt
 endif()
 
-if (MINGW)
-    # Target Windows 8 for PrefetchVirtualMemory
-    add_compile_definitions(_WIN32_WINNT=${GGML_WIN_VER})
-endif()
-
 #
 # build the library
 #