lib: Fix unknown sig_atomic_t compile error

lib: Fix unknown sig_atomic_t compile error

This is happening for Alpine Linux.

```
26 64.59 ./lib/sigevent.h:23:18: error: unknown type name 'sig_atomic_t'
26 64.59    23 |         volatile sig_atomic_t caught; /* private member   */
26 64.59       |                  ^~~~~~~~~~~~
26 64.60 In file included from ./lib/libfrr.h:12,
26 64.60                  from ./lib/vty.h:28,
26 64.60                  from ./lib/command.h:11,
26 64.60                  from ./lib/debug.h:11,
26 64.60                  from ./mgmtd/mgmt.h:12,
26 64.60                  from mgmtd/mgmt_history.c:14:
26 64.60 ./lib/sigevent.h:23:18: error: unknown type name 'sig_atomic_t'
26 64.60    23 |         volatile sig_atomic_t caught; /* private member   */
26 64.60       |                  ^~~~~~~~~~~~
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>