fix: guard activation_finish_absent_publish behind its call-site conditions

Building on FreeBSD after syncing with upstream failed with
-Werror,-Wunused-function on activation_finish_absent_publish in
src/cli/activation_transaction.c.

The function is only invoked from three platform-specific branches:
Windows (_WIN32, MoveFileExW), macOS (__APPLE__, renameatx_np), and
modern Linux (__linux__ with SYS_renameat2, via syscall). On every other
POSIX target — including FreeBSD and other BSDs — control falls through
to the portable linkat(2) fallback, so the helper was defined but never
called, tripping -Wunused-function under -Werror.

Rather than excluding FreeBSD by name (which would still break on
OpenBSD, NetBSD, DragonFly, Solaris, or any Linux without
SYS_renameat2), wrap the function definition in the same #if condition
as its call sites. This keeps the guard mechanically tied to actual
usage instead of enumerating platforms, so it stays correct as new
targets are added.

Signed-off-by: Pedro Ramos <131530838+pr9000@users.noreply.github.com>

Author: PR9000

src/cli/activation_transaction.c

@@ -1695,6 +1695,7 @@ static activation_publish_status_t activation_publish_absent_link_fallback(
 }
 #endif
 
+#if defined(_WIN32) || defined(__APPLE__) || (defined(__linux__) && defined(SYS_renameat2))
 static activation_publish_status_t activation_finish_absent_publish(
     cbm_activation_transaction_t *transaction) {
     transaction->staged_exists = false;
@@ -1705,6 +1706,7 @@ static activation_publish_status_t activation_finish_absent_publish(
                ? ACTIVATION_PUBLISH_OK
                : ACTIVATION_PUBLISH_CHANGED_ERROR;
 }
+#endif
 
 static activation_publish_status_t activation_publish_absent_replacement(
     cbm_activation_transaction_t *transaction) {