The GN was including both libGLESv2.def and _autogen. This fixes the
GN to only include _autogen, and removes the .def include from the
static build target.

Bug: angleproject:2476
Change-Id: I2e856e3bbe875bdadd1add4517cef60a11227047
Reviewed-on: https://chromium-review.googlesource.com/1057451Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Brandon1 Jones <brandon1.jones@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>