Remove a function-like macro and replace it with a function.

[iphydf]

No use making this a macro. LTO will inline this anyway, if we care about
performance. Generally, we avoid function-like macros that can be functions.

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This change is 

[codecov]

Codecov Report

Merging #1208 into master will decrease coverage by 0.2%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #1208     +/-   ##
========================================
- Coverage      83%   82.7%   -0.3%     
========================================
  Files          82      82             
  Lines       14645   14647      +2     
========================================
- Hits        12158   12120     -38     
- Misses       2487    2527     +40

Impacted Files	Coverage Δ
toxcore/state.c	61.1% <0%> (-3.6%)	⬇️
auto_tests/toxav_many_test.c	92.5% <0%> (-4.1%)	⬇️
auto_tests/network_test.c	97.5% <0%> (-2.5%)	⬇️
toxcore/net_crypto.c	92.9% <0%> (-1.6%)	⬇️
toxav/audio.c	65.7% <0%> (-1.5%)	⬇️
toxav/toxav.c	67.4% <0%> (-1.2%)	⬇️
toxcore/group.c	76% <0%> (-0.4%)	⬇️
toxav/msi.c	65.3% <0%> (-0.3%)	⬇️
toxcore/onion_client.c	95.9% <0%> (-0.2%)	⬇️
toxcore/friend_connection.c	94.5% <0%> (+0.5%)	⬆️

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