core/graphicsbuffer: add missing unistd.h include
Hey, I've been packaging Plasma 6 for Chimera Linux (where no Qt5 exists) which uses the musl C standard library and thus ended up building KWin master
and noticing the following build error:
../src/core/shmgraphicsbufferallocator.cpp:105:9: error: use of undeclared identifier 'ftruncate'
if (ftruncate(fd.get(), bufferSize) < 0) {
^
See also: https://man7.org/linux/man-pages/man3/ftruncate.3p.html / https://linux.die.net/man/2/ftruncate