onCmdDirUsage is linux-only for now

Index: src/FilePanel.cpp
--- src/FilePanel.cpp.orig
+++ src/FilePanel.cpp
@@ -128,7 +128,6 @@ FXDEFMAP(FilePanel) FilePanelMap[] =
     FXMAPFUNC(SEL_COMMAND, FilePanel::ID_RUN_SCRIPT, FilePanel::onCmdRunScript),
     FXMAPFUNC(SEL_UPDATE, FilePanel::ID_RUN_SCRIPT, FilePanel::onUpdRunScript),
     FXMAPFUNC(SEL_COMMAND, FilePanel::ID_GO_SCRIPTDIR, FilePanel::onCmdGoScriptDir),
-    FXMAPFUNC(SEL_COMMAND, FilePanel::ID_DIR_USAGE, FilePanel::onCmdDirUsage),
     FXMAPFUNC(SEL_RIGHTBUTTONRELEASE, FilePanel::ID_FILELIST, FilePanel::onCmdPopupMenu),
     FXMAPFUNC(SEL_COMMAND, FilePanel::ID_POPUP_MENU, FilePanel::onCmdPopupMenu),
     FXMAPFUNC(SEL_DOUBLECLICKED, FilePanel::ID_FILELIST, FilePanel::onCmdItemDoubleClicked),
@@ -159,6 +158,7 @@ FXDEFMAP(FilePanel) FilePanelMap[] =
     FXMAPFUNC(SEL_UPDATE, FilePanel::ID_TOGGLE_THUMBNAILS, FilePanel::onUpdToggleThumbnails),
     FXMAPFUNC(SEL_UPDATE, FilePanel::ID_DIR_USAGE, FilePanel::onUpdDirUsage),
 #if defined(linux)
+    FXMAPFUNC(SEL_COMMAND, FilePanel::ID_DIR_USAGE, FilePanel::onCmdDirUsage),
     FXMAPFUNC(SEL_COMMAND, FilePanel::ID_MOUNT, FilePanel::onCmdMount),
     FXMAPFUNC(SEL_COMMAND, FilePanel::ID_UMOUNT, FilePanel::onCmdMount),
     FXMAPFUNC(SEL_UPDATE, FilePanel::ID_MOUNT, FilePanel::onUpdMount),