Add ColumnNothing to the installation script

Additionally, import it in client.h for consistency and to avoid
manually importing it each time this column type is in use.

Author: slabko

clickhouse/CMakeLists.txt

@@ -221,6 +221,7 @@ INSTALL(FILES columns/ip4.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/ip6.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/itemview.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/lowcardinality.h DESTINATION include/clickhouse/columns/)
+INSTALL(FILES columns/nothing.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/nullable.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/numeric.h DESTINATION include/clickhouse/columns/)
 INSTALL(FILES columns/map.h DESTINATION include/clickhouse/columns/)

clickhouse/client.h

@@ -11,6 +11,7 @@
 #include "columns/ip4.h"
 #include "columns/ip6.h"
 #include "columns/lowcardinality.h"
+#include "columns/nothing.h"
 #include "columns/nullable.h"
 #include "columns/numeric.h"
 #include "columns/map.h"