Win32: Fix build with GCC (MinGW)

Cf. https://github.com/godotengine/godot/issues/32384.
diff --git a/basisu_enc.h b/basisu_enc.h
index 05656da..c2b9133 100644
--- a/basisu_enc.h
+++ b/basisu_enc.h
@@ -23,7 +23,7 @@
 #include <thread>
 #include <unordered_map>
 
-#ifndef _WIN32
+#if !defined(_WIN32) || defined(__MINGW32__)
 #include <libgen.h>
 #endif