[lldb] Fix lldb build on musl

Summary: limits.h is needed for getting PATH_MAX definition, this comes to fore
with musl libc where limits.h is not included indirectly via other system headers.

Patch by Khem Raj, thanks!

Reviewers: compnerd

Reviewed By: compnerd

Subscribers: llvm-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D31275

llvm-svn: 340876