Make MGLOpenGLStyleLayer.h public in static library

[1ec5]

Made MGLOpenGLStyleLayer.h public in the static library, fixing a build error that occurred in applications that linked against the static framework.

This issue slipped under our radar through a combination of factors:

• We don’t build an application that links against the static library on Bitrise, or the static library itself for that matter.
• jazzy warns about any symbols declared in the table of contents but left undefined in the framework. However, as of Insert example code in original headers #7569, we point jazzy to the checked-in headers rather than the built static library headers (which omitted MGLOpenGLStyleLayer.h).
• After building the release, I tested linking the dynamic framework into an application but not the static framework.

/cc @kkaefer

[mention-bot]

@1ec5, thanks for your PR! By analyzing this pull request, we identified @boundsj, @incanus and @friedbunny to be potential reviewers.