libxml2 required for test compilation, check for it via meson
Otherwise the build fails like this:
[41/74] Compiling C object 'tests/59830eb@@xmlloader@sta/atk-object-xml-loader.c.o'.
FAILED: tests/59830eb@@xmlloader@sta/atk-object-xml-loader.c.o
gcc -Itests/59830eb@@xmlloader@sta -Itests -I../tests -I. -I../ -Itests/dummyatk -I../tests/dummyatk -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/atk-1.0 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=c99 -O3 -D_DEFAULT_SOURCE -Os -fomit-frame-pointer -Os -fomit-frame-pointer -fPIC -MD -MQ 'tests/59830eb@@xmlloader@sta/atk-object-xml-loader.c.o' -MF 'tests/59830eb@@xmlloader@sta/atk-object-xml-loader.c.o.d' -o 'tests/59830eb@@xmlloader@sta/atk-object-xml-loader.c.o' -c ../tests/atk-object-xml-loader.c
../tests/atk-object-xml-loader.c:26:10: fatal error: libxml/parser.h: No such file or directory
#include <libxml/parser.h>
^~~~~~~~~~~~~~~~~
compilation terminated.
[50/74] Compiling C object 'tests/59830eb@@atk-test@exe/atk_test_accessible.c.o'.