Fix test for wxOSX in wxSTC code

Use "#ifdef", not "#if", to check for __WXMAC__ to avoid warnings about using #if
without argument under the other platforms.

See #16243.

src/stc/ScintillaWX.cpp

@@ -287,7 +287,7 @@ void ScintillaWX::Initialise() {
     // "Cmd" key here, as elsewhere in wx API, while SCI_ALT is the "Option"
     // key (and "Ctrl" key, if we ever need it, should be represented by
     // SCI_META).
-#if __WXMAC__
+#ifdef __WXMAC__
     kmap.AssignCmdKey(SCK_LEFT, SCI_CTRL, SCI_VCHOME);
     kmap.AssignCmdKey(SCK_RIGHT, SCI_CTRL, SCI_LINEEND);
     kmap.AssignCmdKey(SCK_LEFT, SCI_ALT, SCI_WORDLEFT);