Match the Makefile, which never sets LZ4_DEBUG for this test.  If
LZ4_DEBUG is non-zero, lib/lz4.c #includes <assert.h>, the musl version of
which has a conflicting definition for __assert_fail:

    https://www.openwall.com/lists/musl/2019/03/04/6

Fixes build failure:

    ../../../tests/freestanding.c:173:6: error: conflicting types for '__assert_fail'; have 'void(const char *, const char *, unsigned int,  const char *)'
      173 | void __assert_fail(const char * assertion, const char * file, unsigned int line, const char * function) {
          |      ^~~~~~~~~~~~~
    In file included from ../../../tests/../lib/lz4.c:270,
                     from ../../../tests/freestanding.c:49:
    /usr/include/assert.h:19:16: note: previous declaration of '__assert_fail' with type 'void(const char *, const char *, int,  const char *)'
       19 | _Noreturn void __assert_fail (const char *, const char *, int, const char *);
          |                ^~~~~~~~~~~~~