c11/threads: don't include assert.h if the assert macro is already defined

In the gallium code, the assert() macro could come from either the
system's assert.h file (via c11/threads.h) or from gallium's u_debug.h.
It looks like all known assert.h files unconditionally #undef assert
before defining their own version.  So the assert you get depends on
whether threads.h or u_debug.h was included last.

In the gallium code we really want to use the assert() from u_debug.h
(it behaves better on Windows).  In gallium, c11/threads.h is only
included after u_debug.h in the os_thread.h wrapper.  So Adding
an #ifndef assert test in the threads*.h files avoids using the system's
assert().

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

include/c11/threads_posix.h

@@ -27,7 +27,9 @@
  * DEALINGS IN THE SOFTWARE.
  */
 #include <stdlib.h>
+#ifndef assert
 #include <assert.h>
+#endif
 #include <limits.h>
 #include <errno.h>
 #include <unistd.h>

include/c11/threads_win32.h

@@ -26,7 +26,9 @@
  * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  */
+#ifndef assert
 #include <assert.h>
+#endif
 #include <limits.h>
 #include <errno.h>
 #include <process.h>  // MSVCRT