Fix headless build when OpenGL is available #1573

Job	Run time
linux-x86_64-debug	1h 14m 53s
headless	39m 3s
linux (aarch64)	24m 9s
linux (armhf)	30m 40s
linux (i386)	25m 58s
linux (riscv64)	21m 59s
linux (x86_64)	21m 7s
lto	2h 42m 43s
macos (universal-10.15)	2h 2m 21s
modaudio (modduo, modduo-static, MODDUO=true)	39m 35s
modaudio (modduox, modduox-static)	48m 37s
modaudio (moddwarf, moddwarf)	59m 25s
plugin-validation	1h 25m 0s
source-tarball	19m 21s
sysdeps	1h 2m 10s
wasm (simd)	1h 25m 59s
wasm (nosimd)	1h 26m 17s
windows (win32)	35m 49s
windows (win64)	47m 53s
	18h 12m 59s

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