Added search path to Qwt6 finder.

Needed by certain linux distributions.

cmake/FindQwt6.cmake

@@ -35,6 +35,7 @@ find_path(QWT6_INCLUDE_DIR qwt.h
                /opt/local/include
                /opt/local/include/qwt
                /usr/include
+               /usr/include/qwt
                "${VS_SEARCH_PATH}/qwt-6.1.0"
                c:\\libs\\${SEARCHPATH}\\x32\\Qwt-6.1.0
                c:\\libs\\${SEARCHPATH}\\x32\\Qwt-6.0.2