bpo-40268: Remove unused osdefs.h includes (GH-19532)

When the include is needed, add required symbol in a comment.
diff --git a/Modules/_tracemalloc.c b/Modules/_tracemalloc.c
index 691de07..dbae107 100644
--- a/Modules/_tracemalloc.c
+++ b/Modules/_tracemalloc.c
@@ -4,7 +4,6 @@
 #include "pycore_traceback.h"
 #include "hashtable.h"
 #include "frameobject.h"
-#include "osdefs.h"
 
 #include "clinic/_tracemalloc.c.h"
 /*[clinic input]
diff --git a/Modules/getpath.c b/Modules/getpath.c
index 1dd8dd0..94e06b3 100644
--- a/Modules/getpath.c
+++ b/Modules/getpath.c
@@ -4,7 +4,7 @@
 #include "pycore_fileutils.h"
 #include "pycore_initconfig.h"
 #include "pycore_pathconfig.h"
-#include "osdefs.h"
+#include "osdefs.h"               // DELIM
 
 #include <sys/types.h>
 #include <string.h>
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index f5beb09..89f9757 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -312,11 +312,11 @@
 #ifndef IO_REPARSE_TAG_MOUNT_POINT
 #define IO_REPARSE_TAG_MOUNT_POINT (0xA0000003L)
 #endif
-#include "osdefs.h"
+#include "osdefs.h"               // SEP
 #include <malloc.h>
 #include <windows.h>
-#include <shellapi.h>   /* for ShellExecute() */
-#include <lmcons.h>     /* for UNLEN */
+#include <shellapi.h>             // ShellExecute()
+#include <lmcons.h>               // UNLEN
 #define HAVE_SYMLINK
 #endif /* _MSC_VER */
 
diff --git a/Modules/syslogmodule.c b/Modules/syslogmodule.c
index 2451792..11718e2 100644
--- a/Modules/syslogmodule.c
+++ b/Modules/syslogmodule.c
@@ -50,7 +50,7 @@
 /* syslog module */
 
 #include "Python.h"
-#include "osdefs.h"
+#include "osdefs.h"               // SEP
 
 #include <syslog.h>