Suppress error message of execute_process

Command 'execute_process' from OpenCVFindMatlab module can omit error message
to the standard output. This can break build of the OpenCV package in
ExternalProject_Add (Windows/Visual Studio).

cmake/OpenCVFindMatlab.cmake

@@ -94,7 +94,7 @@ function(locate_matlab_root)
     set(REG_EXTENSION_ "SOFTWARE\\Mathworks\\MATLAB")
     set(REG_ROOTS_ "HKEY_LOCAL_MACHINE" "HKEY_CURRENT_USER")
     foreach(REG_ROOT_ ${REG_ROOTS_})
-      execute_process(COMMAND reg query "${REG_ROOT_}\\${REG_EXTENSION_}" OUTPUT_VARIABLE QUERY_RESPONSE_)
+      execute_process(COMMAND reg query "${REG_ROOT_}\\${REG_EXTENSION_}" OUTPUT_VARIABLE QUERY_RESPONSE_ ERROR_VARIABLE UNUSED_)
       if (QUERY_RESPONSE_)
         string(REGEX MATCHALL "[0-9]\\.[0-9]" VERSION_STRINGS_ ${QUERY_RESPONSE_})
         list(APPEND VERSIONS_ ${VERSION_STRINGS_})