Minor: Move make_builder into mod.rs

[lewiszlw]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[alamb]

Thanks for this contribution @lewiszlw -- what is the rationale / reason for this proposed change?

[lewiszlw]

Because make_builder function can make array builders for all data types. It looks a bit unreasonable that putting it in struct_builder.rs file. @alamb

[alamb]

Thank you for the cleanup @lewiszlw

I double checked that this doesn't change the module location of make_builder (it is still in arrow_array) and thus that this is not a breaking API change