Fix: Add DATA_TYPE back to GET_AGGREGATES/AGGREGATE (backport #2236)

[mergify]

What

Add element DATA_TYPE back to element AGGREGATE in GMP GET_AGGREGATES.

Why

Element was accidentally removed.

References

This element was removed in d73fba8 (SVN r23003) in 2015. In omp.c line 16809 was removed from omp_xml_handle_end_element but was missing from the addition buffer_aggregate_xml at line 11416.

It looks like this was an accident because the element is still in the GMP doc, and the removal would likely have been mentioned in the commit log. Let me know if this was intentional though, I'll adjust the doc instead.

Note that the original had a type NULL check before the g_string_append_printf, but type is already checked at the top of handle_get_aggregates.

Quick test

Before PR:

$ o m m '<get_aggregates type="cve" group_column="severity"/>'
<get_aggregates_response status_text="OK" status="200">
  <aggregate>
    <group_column>severity</group_column>

$ o m m '<get_aggregates type="result" group_column="description" mode="word_counts" max_groups="10"/>'
<get_aggregates_response status_text="OK" status="200">
  <aggregate>
    <group_column>description</group_column>

After PR, element data_type is back:

$ o m m '<get_aggregates type="cve" group_column="severity"/>'
<get_aggregates_response status_text="OK" status="200">
  <aggregate>
    <data_type>cve</data_type>
    <group_column>severity</group_column>

$ o m m '<get_aggregates type="result" group_column="description" mode="word_counts" max_groups="10"/>'
<get_aggregates_response status_text="OK" status="200">
  <aggregate>
    <data_type>result</data_type>
    <group_column>description</group_column>
```<hr>This is an automatic backport of pull request #2236 done by [Mergify](https://mergify.com).

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Bug Fixes	1

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