Fix WIN32 define timing in IGFD

When the compiler hits IGFD, WIN32 is only set if standard extensions are enabled. (Why?)
IGFD identifies Windows via WIN32 instead of _WIN32. (Why?)

- WIN32 is needed for #ifdef-gated Windows-specific methods in the header & code files.
  - There's fallback code in the code file to set WIN32 if _WIN32 or __WIN32__ is set.
    - Which is hit *after* the header gets included, so the header doesn't expose required methods.
      -> Boom, error!

Workaround by moving the fallback into the header.

extern/igfd/ImGuiFileDialog.cpp

@@ -44,10 +44,7 @@ SOFTWARE.
 #if defined (__EMSCRIPTEN__) // EMSCRIPTEN
 	#include <emscripten.h>
 #endif // EMSCRIPTEN
-#if defined(__WIN32__) || defined(_WIN32)
-	#ifndef WIN32
-		#define WIN32
-	#endif // WIN32
+#ifdef WIN32
 	#define stat _stat
 	#define stricmp _stricmp
 	#include <cctype>

extern/igfd/ImGuiFileDialog.h

@@ -581,6 +581,12 @@ ImGuiFontStudio is using also ImGuiFileDialog.
 #ifndef IMGUIFILEDIALOG_H
 #define IMGUIFILEDIALOG_H
 
+#if defined(__WIN32__) || defined(_WIN32)
+	#ifndef WIN32
+		#define WIN32
+	#endif // WIN32
+#endif // defined(__WIN32__) || defined(_WIN32)
+
 #define IMGUIFILEDIALOG_VERSION "v0.6.4"
 
 #ifndef CUSTOM_IMGUIFILEDIALOG_CONFIG