macOS headers: add memory.h
macOS headers: add memory.h

it's provided by macOS LibC and apparently there is code in the wild
that depends on it. all it does is #include <string.h>

closes #7249

zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig