Fix NetBSD build with CMake 3.5.2

Summary:
Building HEAD of LLDB fails in linking against DebugInfoPDB. It also prints the following warning:

```
CMake Warning (dev) in source/Plugins/SymbolFile/PDB/CMakeLists.txt:
  Policy CMP0022 is not set: INTERFACE_LINK_LIBRARIES defines the link
  interface.  Run "cmake --help-policy CMP0022" for policy details.  Use the
  cmake_policy command to set the policy and suppress this warning.

  Target "lldbPluginSymbolFilePDB" has an INTERFACE_LINK_LIBRARIES property.
  This should be preferred as the source of the link interface for this
  library but because CMP0022 is not set CMake is ignoring the property and
  using the link implementation as the link interface instead.

  INTERFACE_LINK_LIBRARIES:

    LLVMDebugInfoPDB

  Link implementation:

    (empty)
```

CMP0022 was introduced in CMake-2.8.11, bump minimal required version from 2.8 to 3.0 to gain more useful features like libexecinfo(3) detection on NetBSD.

Reviewers: emaste, zturner, labath

Subscribers: zturner, lldb-commits, joerg

Differential Revision: http://reviews.llvm.org/D19685

llvm-svn: 268191