Use (void) for empty parameter lists in C.

[iphydf]

Because () means "some unknown number of parameters".

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This change is 

[sudden6]

Reviewed 3 of 3 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @Diadlo)

[codecov]

Codecov Report

Merging #1150 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1150     +/-   ##
========================================
- Coverage    82.8%   82.7%   -0.2%     
========================================
  Files          82      82             
  Lines       14667   14594     -73     
========================================
- Hits        12158   12074     -84     
- Misses       2509    2520     +11

Impacted Files	Coverage Δ
auto_tests/dht_test.c	48% <100%> (ø)	⬆️
auto_tests/onion_test.c	86.5% <100%> (ø)	⬆️
auto_tests/TCP_test.c	100% <100%> (ø)	⬆️
toxav/toxav.c	68.1% <0%> (-1%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxcore/Messenger.c	85.8% <0%> (-0.8%)	⬇️
toxcore/net_crypto.c	93% <0%> (-0.5%)	⬇️
toxcore/group.c	76% <0%> (-0.3%)	⬇️
toxcore/onion_client.c	95.2% <0%> (-0.2%)	⬇️
toxav/msi.c	65.6% <0%> (ø)	⬆️
... and 3 more

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[nurupo]

Reviewed 3 of 3 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @Diadlo)