Don't include sys/param.h for _BYTE_ORDER

[giannitedesco]

It includes a bunch of other files including a lot of macros which can
reduce the namespace available for actual protobuf.

For example, create a protobuf with a member called SIGSEGV. Since
macros cannot be namespaced in C++ this results in code which cannot be
compiled.

Fix this by just directly including endian.h

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[giannitedesco]

@googlebot I signed it!

[acozzette]

It looks like this endian.h header doesn't exist on Mac OS unfortunately.

[giannitedesco]

Ah thanks. Apparently machine/endian.h is the thing to include on apple implementations.

[acozzette]

Thanks, @giannitedesco.