gl_rasterizer: Silence implicit truncation warning in SetupShaders()

Previously this would warn of truncating a std::size_t to a u32. This is
safe because we'll obviously never have more than UINT32_MAX amount of
uniform buffers.

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -211,7 +211,7 @@ void RasterizerOpenGL::SetupShaders(u8* buffer_ptr, GLintptr buffer_offset) {
 
     // Next available bindpoints to use when uploading the const buffers and textures to the GLSL
     // shaders. The constbuffer bindpoint starts after the shader stage configuration bind points.
-    u32 current_constbuffer_bindpoint = uniform_buffers.size();
+    u32 current_constbuffer_bindpoint = static_cast<u32>(uniform_buffers.size());
     u32 current_texture_bindpoint = 0;
 
     for (size_t index = 0; index < Maxwell::MaxShaderProgram; ++index) {