gh-157899: Define HAVE_FREADLINK_RUNTIME on FreeBSD 16 (GH-159397)

The commit 4b0e334843b76043c8e69b16ceb4c8478f134c01 added support for
freadlink() on macOS. The function is also available on FreeBSD 16.
Define HAVE_FREADLINK_RUNTIME on FreeBSD.

Modules/posixmodule.c

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index a3f4449..dfc5409 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -644,6 +644,7 @@ static const unsigned int _Py_STATX_KNOWN = (STATX_BASIC_STATS | STATX_BTIME
 #  define HAVE_PTSNAME_R_RUNTIME 1
 #  define HAVE_DUP3_RUNTIME 1
 #  define HAVE_PIPE2_RUNTIME 1
+#  define HAVE_FREADLINK_RUNTIME 1
 #endif