Fix build of "tests" on macOS

Building the C++ unit tests failed for
 tests/src/Mod/Sketcher/App/SketchObject.cpp
because some XML headers were not found. This was because the XercesC
include dir was not in the search list for headers.

Details:
> In file included from /Users/jonas/src/FreeCAD/FreeCAD-git/tests/src/Mod/Sketcher/App/SketchObject.cpp:7:
> In file included from /Users/jonas/src/FreeCAD/FreeCAD-git/src/App/Application.h:33:
> /Users/jonas/src/FreeCAD/FreeCAD-git/src/Base/Parameter.h:55:10: fatal error: 'xercesc/util/XercesDefs.hpp' file not found
> #include <xercesc/util/XercesDefs.hpp>
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is the same issue as already adresses previously with PR9657 [1] but
the test for SketchObject was added later [2].

[1]: FreeCAD#9657
[2]: FreeCAD@532b391

tests/CMakeLists.txt

@@ -54,6 +54,7 @@ target_include_directories(Sketcher_tests_run PUBLIC
     ${EIGEN3_INCLUDE_DIR}
     ${OCC_INCLUDE_DIR}
     ${Python3_INCLUDE_DIRS}
+    ${XercesC_INCLUDE_DIRS}
 )
 target_link_libraries(Sketcher_tests_run
     gtest_main