tls: Include <poll.h> instead of <sys/poll.h>

musl libc warns about these:
```
In file included from src/tls/socket-io.c:32:
/usr/x86_64-pc-linux-musl/include/sys/poll.h:1:2: warning: redirecting incorrect #include <sys/poll.h> to <poll.h> [-W#warnings]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^
  CC       src/pam-ssh-add/pam_ssh_add_so-pam-ssh-add.o
In file included from src/tls/connection.c:37:
/usr/x86_64-pc-linux-musl/include/sys/poll.h:1:2: warning: redirecting incorrect #include <sys/poll.h> to <poll.h> [-W#warnings]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^
```
and glibc's `<sys/poll.h>` also just wraps `<poll.h>`. Use `<poll.h>` to get
rid of the warning on musl based systems

src/tls/connection.c

@@ -28,13 +28,13 @@
 #include <fcntl.h>
 #include <net/if.h>
 #include <netinet/in.h>
+#include <poll.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/epoll.h>
 #include <sys/param.h>
-#include <sys/poll.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/timerfd.h>

src/tls/socket-io.c

@@ -26,10 +26,10 @@
 #include <err.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <poll.h>
 #include <stdarg.h>
 #include <stdint.h>
 #include <stdio.h>
-#include <sys/poll.h>
 #include <sys/socket.h>
 #include <sys/un.h>