bpo-40190: Add support for _SC_AIX_REALMEM in sysconf (GH-19380)

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 1adca8e..345798d 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -11424,6 +11424,9 @@
 #ifdef _SC_PAGE_SIZE
     {"SC_PAGE_SIZE",    _SC_PAGE_SIZE},
 #endif
+#ifdef _SC_AIX_REALMEM
+    {"SC_AIX_REALMEM", _SC_AIX_REALMEM},
+#endif
 #ifdef _SC_PASS_MAX
     {"SC_PASS_MAX",     _SC_PASS_MAX},
 #endif