compat: fix basename buffer to be thread-local on Windows

Add `__declspec(thread)` to the static buffer in the Windows
`basename` func to prevent data races in multi-threaded
contexts, where concurrent calls could corrupt a shared static buffer.

[__declspec(thread)](https://learn.microsoft.com/en-us/cpp/cpp/thread?view=msvc-170)

Signed-off-by: slabbancz <slabbancz@microsoft.com>

Author: slab-msft

include/fluent-bit/flb_compat.h

@@ -109,7 +109,7 @@ static inline char* basename(const char *path)
     char dir[_MAX_DIR];
     char fname[_MAX_FNAME];
     char ext[_MAX_EXT];
-    static char buf[_MAX_PATH];
+    static __declspec(thread) char buf[_MAX_PATH];
 
     _splitpath_s(path, drive, _MAX_DRIVE, dir, _MAX_DIR,
                        fname, _MAX_FNAME, ext, _MAX_EXT);