fix C API includes

ColumnStore used to include server's mysql.h
but link all tools with libmariadb.so

There's no guarantee that this would work, even with workarounds
it had in dbcon/mysql/sm.cpp

Fix:
* tools (linked with libmariadb.so) *must* include libmariadb's mysql.h
* as a hack prevent service_thd_timezone.h from being loaded into tools,
  as it conflicts with libmariadb's mysql.h
* server plugin *must* include server's mysql.h
* also don't link every tool with libmariadb.so, link the helper library
  (liblibmysqlclient.so) that actually needs it, tools use this
  helper library, not libmariadb.so directly

dbcon/mysql/sm.cpp

@@ -506,5 +506,4 @@ void cpsm_conhdl_t::write(ByteStream bs)
 #endif
 }
 
-}  // namespace sm
-
+}