Stop ultralib's __assert declaration colliding with Darwin's macro

[buddingmonkey]

The problem

Apple's <assert.h> already provides __assert, but as a three-argument
function-like macro, not a function:

#define __assert(e, file, line) __assert_rtn(..., file, line, e)

lib/ultralib/include/ultra_assert.h declares its own:

extern void __assert(const char *, const char *, int);

On Apple platforms the macro expands mid-declaration and the parse collapses,
taking the rest of the header down with it.

Why this hasn't been noticed

The declaration lives in the #else (i.e. !NDEBUG) branch of the header.
Release builds define NDEBUG, take the #define assert(EX) ((void)0) branch
above, and never reach the declaration. Only Debug builds break.

The fix

Declare __assert only where the platform has not already provided it:

#ifndef __assert
extern void __assert(const char *, const char *, int);
#endif

The macro's argument order (expression, file, line) matches the call sites
further down this same header, so where the platform supplies __assert we let
it handle the failure — on Apple platforms assert() now routes through
__assert_rtn. Behaviour on every other platform is unchanged: nothing else
defines __assert as a macro, so the declaration is emitted exactly as before.

One file, ten added lines (eight of which are the explanatory comment).

Notes

Not covered by run-clang-format.sh (which only formats src/port), so no
formatting gate applies here — confirmed by running the script and seeing the
tree stay clean apart from this change.

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Prepared with AI assistance; the change and its rationale were verified by hand.