package/nut: ensure time_t is defined

Ensure time_t is defined to fix the following collectd build failure on
musl:

src/nut.c:40:2: error: #error "Unable to determine the UPS connection type."
   40 | #error "Unable to determine the UPS connection type."
      |  ^~~~~
src/nut.c:46:3: error: unknown type name 'collectd_upsconn_t'
   46 |   collectd_upsconn_t *conn;
      |   ^~~~~~~~~~~~~~~~~~

libupsclient is an optional dependency of nut plugin since version
5.10.0 of collectd and
https://github.com/collectd/collectd/commit/bc2d94024d0250211c1e498db1b43bb4525324da

Fixes:
 - http://autobuild.buildroot.org/results/22b758097e8fb72c68e41329cbc7abc748d81ca6



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>