spl: fix compilation without HAVE_BACKTRACE

The __maybe_unused macro is defined in spl/sys/debug.h

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #16229

lib/libspl/backtrace.c

@@ -110,6 +110,8 @@ libspl_backtrace(int fd)
 	backtrace_symbols_fd(btptrs, nptrs, fd);
 }
 #else
+#include <sys/debug.h>
+
 void
 libspl_backtrace(int fd __maybe_unused)
 {