posix: syslog: Fix uninitialized variable error

Seen this in compiler

lib/posix/options/syslog.c: In function 'setlogmask':
lib/posix/options/syslog.c:66:16: error: 'oldpri' may be used uninitialized
   66 |         return oldpri;
      |                ^~~~~~
lib/posix/options/syslog.c:59:13: note: 'oldpri' was declared here
   59 |         int oldpri;
      |             ^~~~~~
lib/posix/options/syslog.c: In function 'vsyslog':
lib/posix/options/syslog.c:83:33: error: 'mask' may be used uninitialized
   83 |         if ((BIT(level) & mask) == 0) {
      |             ~~~~~~~~~~~~~~~~~~~~^~~~
lib/posix/options/syslog.c:71:17: note: 'mask' was declared here
   71 |         uint8_t mask;
      |                 ^~~~

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>