[libcxx] Include <sys/time.h> in posix_compat.h

posix_compat.h uses struct timeval which is defined in <sys/time.h>
but it doesn't include it. On most POSIX platforms like Linux or macOS,
that headers is transitively included by other headers like <sys/stat.h>,
but there are other platforms where this is not the case.

Differential Revision: https://reviews.llvm.org/D153384