expat: Update to version 2.4.2

expat: Update to version 2.4.2

- Update from 2.4.1 to 2.4.2
- Update of rootfile
- Changelog
   Release 2.4.2 Sun December 19 2021
     Other changes:
       #509 #510  Link againgst libm for function "isnan"
       #513 #514  Include expat_config.h as early as possible
            #498  Autotools: Include files with release archives:
                    - buildconf.sh
                    - fuzz/*.c
       #507 #519  Autotools: Sync CMake templates
       #495 #524  CMake: MinGW: Fix pkg-config section "Libs" for
                    - non-release build types (e.g. -DCMAKE_BUILD_TYPE=Debug)
                    - multi-config CMake generators (e.g. Ninja Multi-Config)
       #502 #503  docs: Document that function XML_GetBuffer may return NULL
                    when asking for a buffer of 0 (zero) bytes size
       #522 #523  docs: Fix return value docs for both
                    XML_SetBillionLaughsAttackProtection* functions
       #525 #526  Version info bumped from 9:1:8 to 9:2:8;
                    see https://verbump.de/ for what these numbers do

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>