Use _WIN32 instead of WIN32 to detect Windows

`WIN32` is not defined when building a 64-bit executable on Windows with Clang. `_WIN32` is the more appropriate option here.
http://web.archive.org/web/20191012035921/http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system contains a handy table/matrix of the options and why this is best.

rtgui/filepanel.cc

@@ -26,7 +26,7 @@
 #include "placesbrowser.h"
 #include "thumbnail.h"
 
-#ifdef WIN32
+#ifdef _WIN32
 #include "windows.h"
 #endif
 
@@ -302,7 +302,7 @@ bool FilePanel::imageLoaded( Thumbnail* thm, ProgressConnector<rtengine::Initial
             if (options.tabbedUI) {
                 EditorPanel* epanel;
                 {
-#ifdef WIN32
+#ifdef _WIN32
                     int winGdiHandles = GetGuiResources( GetCurrentProcess(), GR_GDIOBJECTS);
                     if(winGdiHandles > 0 && winGdiHandles <= 6500) //(old settings 8500) 0 means we don't have the rights to access the function, 8500 because the limit is 10000 and we need about 1500 free handles
                     //J.Desmis october 2021 I change 8500 to 6500..Why ? because without while increasing size GUI system crash in multieditor
@@ -312,7 +312,7 @@ bool FilePanel::imageLoaded( Thumbnail* thm, ProgressConnector<rtengine::Initial
                     epanel = Gtk::manage (new EditorPanel ());
                     parent->addEditorPanel (epanel, pl->thm->getFileName());
                     }
-#ifdef WIN32
+#ifdef _WIN32
                     else {
                         Glib::ustring msg_ = Glib::ustring("<b>") + M("MAIN_MSG_CANNOTLOAD") + " \"" + escapeHtmlChars(thm->getFileName()) + "\" .\n" + M("MAIN_MSG_TOOMANYOPENEDITORS") + "</b>";
                         Gtk::MessageDialog msgd (*parent, msg_, true, Gtk::MESSAGE_ERROR, Gtk::BUTTONS_OK, true);
@@ -339,7 +339,7 @@ bool FilePanel::imageLoaded( Thumbnail* thm, ProgressConnector<rtengine::Initial
             Gtk::MessageDialog msgd (*parent, msg_, true, Gtk::MESSAGE_ERROR, Gtk::BUTTONS_OK, true);
             msgd.run ();
         }
-#ifdef WIN32
+#ifdef _WIN32
 MAXGDIHANDLESREACHED:
 #endif
         delete pl->pc;