Add darwin support to NDB.

[rb2k]

This seems to do the trick for getting NDB to compile on macOS.
We've been running this patch in production for a bit and it hasn't caused any issues for us.

Feedback welcome, My C is very very rusty, so there might be better ways of doing this :)

[mlschroe]

Is there really no header file that defines htole32()/le32toh()? And if that's not the case wouldn't

#define htole32(x) OSSwapHostToLittleInt32(x)
#define le32toh(x) OSSwapLittleToHostInt32(x)

be cleaner?

[rb2k]

@mlschroe ah yes, that makes sense. I added it in the same ifdef __APPLE__ block as the endian.h. Not sure if there's a cleaner way.

I don't think there's a header with those two functions on darwin, at least I see references to that in e.g. https://lists.denx.de/pipermail/u-boot/2014-October/192892.html and patches like https://cgit.freedesktop.org/xcb/util-cursor/commit/?id=f03cc278c6cce0cf721adf9c3764d3c5fba63392

[pmatilai]

Thanks for the patch!