Add 'struct' before halide_metal_device
Add 'struct' before halide_metal_device

Compiling Objective-C files without C++ enabled will complain otherwise


Former-commit-id: 443665c168bdd4d99772b0852fa274798431c194
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide