libxml2: upgrade 2.13.5 -> 2.13.6

libxml2: upgrade 2.13.5 -> 2.13.6

https://gitlab.gnome.org/GNOME/libxml2/-/releases/v2.13.6

Release notes
Security

    [CVE-2025-24928] Fix stack-buffer-overflow in xmlSnprintfElements
    [CVE-2024-56171] Fix use-after-free after xmlSchemaItemListAdd
    pattern: Fix compilation of explicit child axis

Regressions

    xmllint: Support compressed input from stdin
    uri: Fix handling of Windows drive letters
    reader: Fix return value of xmlTextReaderReadString again
    SAX2: Fix xmlSAX2ResolveEntity if systemId is NULL

Portability

    dict: Handle ENOSYS from getentropy gracefully
    Fix compilation with uclibc (Dario Binacchi)
    python: Declare init func with PyMODINIT_FUNC
    tests: Fix sanitizer version check on old Apple clang
    cmake: Work around broken sys/random.h in old macOS SDKs

Build

    autotools: Set AC_CONFIG_AUX_DIR
    cmake: Always build Python module as shared library
    cmake: add missing Bcrypt link on Windows (Saleem Abdulrasool)
    cmake: Fix compatibility in package version file

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>