Add some const qualifiers

[zugz]

(Implementing suggestions of sudden6)

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This change is 

[iphydf]

Looks good, module one nit.

[iphydf]

You can also change the (Tox *) to (const Tox *) for consistency.

[codecov]

Codecov Report

Merging #1296 into master will decrease coverage by 0.2%.
The diff coverage is 67.5%.

@@           Coverage Diff            @@
##           master   #1296     +/-   ##
========================================
- Coverage    83.2%     83%   -0.3%     
========================================
  Files          82      82             
  Lines       14978   14962     -16     
========================================
- Hits        12469   12425     -44     
- Misses       2509    2537     +28

Impacted Files	Coverage Δ
toxcore/tox.c	63.6% <67.4%> (ø)	⬆️
toxcore/group.c	78.8% <67.7%> (ø)	⬆️
toxav/msi.c	63.6% <0%> (-4.8%)	⬇️
toxcore/TCP_client.c	64.6% <0%> (-2.9%)	⬇️
toxcore/TCP_server.c	80.3% <0%> (-2.6%)	⬇️
toxav/toxav.c	66.6% <0%> (-2.5%)	⬇️
toxcore/Messenger.c	86.7% <0%> (-0.3%)	⬇️
toxcore/onion_client.c	96% <0%> (ø)	⬆️
toxcore/net_crypto.c	94.9% <0%> (+0.9%)	⬆️
auto_tests/toxav_many_test.c	96.6% <0%> (+4%)	⬆️
... and 1 more

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