Include <sstream> explicitly, and don't rely on the implicit include via <complex>.

This implicit dependency does no longer exist in a recent llbm release (sha 78be61871704).

(cherry picked from commit f0ce88cff75e2020b4beae54b094c77c526724a2)

Bug: none
Test: treehugger
Change-Id: I99de5fa44b488209f60cec59902574e0f91cbce0