modernize header guards to #pragma once

libobs-d3d11/d3d11-exports.h

libobs-opengl/gl-exports.h

libobs-opengl/gl-helpers.h

libobs-opengl/gl-shaderparser.h

libobs-opengl/gl-subsystem.h

libobs/graphics/axisang.h

libobs/graphics/bounds.h

libobs/graphics/effect-parser.h

libobs/graphics/effect.h

libobs/graphics/graphics-internal.h

libobs/graphics/graphics.h

libobs/graphics/input.h

libobs/graphics/math-defs.h

libobs/graphics/math-extra.h

libobs/graphics/matrix3.h

libobs/graphics/matrix4.h

libobs/graphics/plane.h

libobs/graphics/quat.h

libobs/graphics/shader-parser.h

libobs/graphics/vec2.h