libpixelflinger: Fix -Woverloaded-virtual warnings.

GGLAssembler inherits a reset method from two parent classes, but it
provides a separate reset method with a different signature, thus hiding
the inherited reset methods.

The fix is to explicitly hide the inherited reset methods as private.

Bug: 27346663
Change-Id: Ic64b00acb6d77fb791cfdeade9ef7e20148817c1