Fix compilation on libmusl. (#4686)
Fix compilation on libmusl. (#4686)

`blksize_t` is only defined in `sys/types.h`.

graetzer/arangodb,wiltonlazary/arangodb,arangodb/arangodb,fceller/arangodb,arangodb/arangodb,graetzer/arangodb,wiltonlazary/arangodb,graetzer/arangodb,graetzer/arangodb,graetzer/arangodb,Simran-B/arangodb,wiltonlazary/arangodb,arangodb/arangodb,fceller/arangodb,Simran-B/arangodb,Simran-B/arangodb,wiltonlazary/arangodb,Simran-B/arangodb,graetzer/arangodb,fceller/arangodb,Simran-B/arangodb,graetzer/arangodb,graetzer/arangodb,graetzer/arangodb,wiltonlazary/arangodb,graetzer/arangodb,arangodb/arangodb,fceller/arangodb,wiltonlazary/arangodb,arangodb/arangodb,fceller/arangodb,Simran-B/arangodb,arangodb/arangodb,arangodb/arangodb,Simran-B/arangodb,graetzer/arangodb,fceller/arangodb,wiltonlazary/arangodb,graetzer/arangodb,graetzer/arangodb,fceller/arangodb,arangodb/arangodb,Simran-B/arangodb,wiltonlazary/arangodb,fceller/arangodb,fceller/arangodb,Simran-B/arangodb,graetzer/arangodb,fceller/arangodb,Simran-B/arangodb