Add -lpthread to LLDB shared lib link line unconditionally

Usually -lpthread is included due to LLVM link options,
but when LLVM threading is disabled, this does not happen.
pthread is still needed however because LLDB uses threading
regardless of whether LLVM is built with threading support or not.

Differential Revision: http://reviews.llvm.org/D5431

llvm-svn: 241006