Update to obs-studio 28.1.2 in buildspec file

[flying19880517]

Description

Update to obs-studio 28.1.2 in buildspec file. Obs-studio add pthread.h as public header in new versions, obs-studio version in buildspec need to update.

Motivation and Context

Fix https://github.com/obsproject/obs-plugintemplate/issues/36

How Has This Been Tested?

Tested with a plugin template build on windows including <util/threading.h> from libobs.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[gxalpha]

There is https://github.com/obsproject/obs-plugintemplate/pull/76 which updates the entire build system, part of which updates OBS to 29.1.