core/graphicsbuffer: add missing unistd.h include

Fixes an error: use of undeclared identifier 'ftruncate' under Chimera
Linux which uses the musl C standard library.

src/core/shmgraphicsbufferallocator.cpp

@@ -13,6 +13,7 @@
 
 #include <drm_fourcc.h>
 #include <fcntl.h>
+#include <unistd.h>
 #include <sys/mman.h>
 
 namespace KWin