fix lldb-gtest target of Xcode build

Change r278527 was filtering out too many libraries.
The Xcode lldb-gtest target depends on linking libgtest*.a,
but those were not being included.  This caused the lldb-gtest
linkage step to fail to find a main entry point that is present
in the filtered out libs.

This change restores the libgtest* libraries to the link list
by whitelisting them in the filter.

llvm-svn: 278552