added aucio pcma support

[slabajo]

What is the current behavior you want to change?

We want to add PCMA support to WebRTC and RTP that was initially disabled

What is the new behavior provided by this change?

Just adds the audio PCMA codec to the available ones in SDP negotiation

How has this been tested?

Using standard Kurento tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature / enhancement (non-breaking change which improves the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change requires a change to the documentation
• My change requires a change in other repository

Checklist

• I have read the Contribution Guidelines
• I have added an explanation of what the changes do and why they should be included
• I have written new tests for the changes, as applicable, and have successfully run them locally