i915: Fix type of "specoffset" variable.

i915: Fix type of "specoffset" variable.

Commit 2e5a1a2 (intel: Convert from GLboolean to 'bool' from
stdbool.h.) converted the "specoffset" local variable (in
intel_tris.c) from a GLboolean to a bool.  However, GLboolean was the
wrong type for specoffset--it should have been a GLuint (to match the
declaration of specoffset in struct intel_context).

This patch changes specoffset to the proper type.

Fixes piglit test general/two-sided-lighting-separate-specular.

This is a candidate for stable branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45917
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>