Fix multi-process-driver.cpp build on NetBSD

Summary:
Include <string.h> for strcmp(3) and memset(3).

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, clayborg, emaste, labath

Reviewed By: clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29403

llvm-svn: 293895