Fix build error: initializer element is not constant

[jenav]

Fixes #240

[codecov]

Codecov Report

Merging #241 into next will decrease coverage by 0.83%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             next     #241      +/-   ##
==========================================
- Coverage   31.32%   30.48%   -0.84%     
==========================================
  Files          45       45              
  Lines        8167     8168       +1     
==========================================
- Hits         2558     2490      -68     
- Misses       5609     5678      +69

Impacted Files	Coverage Δ
src/common.h	4.76% <0%> (-47.62%)	⬇️
src/opengl.h	16.66% <0%> (-40.48%)	⬇️
src/x.h	52.63% <0%> (-26.32%)	⬇️
src/log.h	68.75% <0%> (-12.5%)	⬇️
src/x.c	37.28% <0%> (-5.23%)	⬇️
src/event.c	36.69% <0%> (-2.7%)	⬇️
src/win.c	48.96% <0%> (-0.6%)	⬇️
src/compton.c	60.19% <0%> (-0.4%)	⬇️

[yshui]

Thanks!