build: fix compilation issue on OpenBSD

During our development, we modified the way
we report backtraces.

On a minimal configuration in OpenBSD, it seems that we
no longer compile from commit a9f26b7 because
our conditional code is buggy.

With the following compiler

vultr# cc -v
OpenBSD clang version 13.0.0
Target: amd64-unknown-openbsd7.3
Thread model: posix
InstalledDir: /usr/bin

We have the following error

cc common/channel_id.c
cc common/daemon.c
common/daemon.c:218:2: error: implicit declaration of function 'add_steal_notifiers' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        add_steal_notifiers(NULL);
        ^
1 error generated.
gmake: *** [Makefile:298: common/daemon.o] Error 1

Reported-by: @grubles
Fixes a9f26b7
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

common/daemon.c

@@ -136,6 +136,10 @@ const char *backtrace_symname(const tal_t *ctx, const void *addr)
 {
 	return "unknown (backtrace unsupported)";
 }
+
+static void add_steal_notifiers(const tal_t *root)
+{
+}
 #endif
 
 int daemon_poll(struct pollfd *fds, nfds_t nfds, int timeout)