include fcntl.h on all *nix platforms (closes #24217)

Patch by Jeffrey Armstrong.
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 0784350..8fdf7f7 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -23,9 +23,9 @@
 
 #ifndef MS_WINDOWS
 #define UNIX
-# ifdef __APPLE__
+# ifdef HAVE_FCNTL_H
 #  include <fcntl.h>
-# endif
+# endif /* HAVE_FCNTL_H */
 #endif
 
 #ifdef MS_WINDOWS