Remove csrc from the RTPHeader struct.

[iphydf]

This is not used by anything in the code, so we shouldn't have it in the
header.

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This change is 

[Diadlo]

Why not make gtest an optional dependency and make this test an optional in cmake?

[sudden6]

removal don't know about the tests

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Review status: 0 of 7 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[sudden6]

Reviewed 2 of 7 files at r1.
Review status: 2 of 7 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[iphydf]

Added gtest to cmake. Not running it on travis.

[nurupo]

Could we not make C++ compiler required for building toxcore without having a very good reason for this? :\

[iphydf]

It's not required for compiling it, but maybe for compiling its tests.

[iphydf]

(some of the tests).

[nurupo]

It does make sense for the monolith test, but not much for others. Why is rtp_test.cpp in C++? Why do we use gtest when we use libcheck in all other tests? This looks very out of place in toxcore.

[iphydf]

Why C++ and gtest: it's much easier to write unit tests with it. libcheck is not much more than a glorified printf debugging library. Comparing values, printing them, etc. is spartanic. I'm not going to write more tests with libcheck, I'd rather get rid of it altogether.

Given the opposition, I've removed it from cmake again.

[iphydf]

gtest is back in #757, and libcheck is gone in #766.