select: Include "grn_ctx.hpp" even if in an environment without Apache Arrow. (#1374)
select: Include "grn_ctx.hpp" even if in an environment without Apache Arrow. (#1374)

Because we use "grn::ChildCtxReleaser" in "proc_select.cpp".
ChildCtxReleaser class is defined in "grn_ctx.hpp".
Therefore, if we don't include "grn_ctx.hpp", a build error occurs in
an environment without Apache Arrow.
groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga