Attempt to fix the presumed problem

Author: Thomas Kiley

regression/validate-trace-xml-schema/CMakeLists.txt

@@ -20,7 +20,7 @@ else()
   if(NOT ${PYTHONINTERP_FOUND})
     message(WARNING "Python >= 3.5 not found, trace schema tests will fail")
   endif()
-  if(NOT XMLLINT)
+  if(NOT (${XMLLINT}))
     message(WARNING "xmllint not found, trace schema tests will fail")
   endif()
 endif()