Support new slicer header location

The location of slicer headers has changed. Since this is an external
project we will just add the new location to the include path for now
until upstream can be updated.

Bug: 65422458
Test: build.
Test: cts-tradefed run cts -m CtsInlineMockingTestCases
Change-Id: I2b79cf5e9b8fd89ae26cdeb0143c31479f00037a