lightningd: fix compile error on unused variable

Switch to write_all instead

Error on gcc 7.3.0:

lightningd/lightningd.c: In function ‘on_sigterm’:
lightningd/lightningd.c:587:9: error: ignoring return value of ‘write’, declared
  with attribute warn_unused_result [-Werror=unused-result]
         write(STDERR_FILENO, msg, strlen(msg));
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: William Casarin <jb55@jb55.com>

lightningd/lightningd.c

@@ -577,14 +577,14 @@ void notify_new_block(struct lightningd *ld, u32 block_height)
 static void on_sigint(int _ UNUSED)
 {
         static const char *msg = "lightningd: SIGINT caught, exiting.\n";
-        write(STDERR_FILENO, msg, strlen(msg));
+        write_all(STDERR_FILENO, msg, strlen(msg));
         _exit(1);
 }
 
 static void on_sigterm(int _ UNUSED)
 {
         static const char *msg = "lightningd: SIGTERM caught, exiting.\n";
-        write(STDERR_FILENO, msg, strlen(msg));
+        write_all(STDERR_FILENO, msg, strlen(msg));
         _exit(1);
 }