Fix build failure with --disable-stats

[voetsjoeba]

A build with ./configure --disable-stats currently fails with the following error (gcc 4.8.5 on CentOS 6):

nc_server.c: In function ‘server_failure’:
nc_server.c:291:5: warning: implicit declaration of function ‘stats_server_set_ts’ [-Wimplicit-function-declaration]
     stats_server_set_ts(ctx, server, server_ejected_at, now);
     ^
nc_server.c:291:38: error: ‘server_ejected_at’ undeclared (first use in this function)
     stats_server_set_ts(ctx, server, server_ejected_at, now);
                                      ^
nc_server.c:291:38: note: each undeclared identifier is reported only once for each function it appears in

The reason appears to be that some functions are not being nopped out in nc_stats.h when NC_STATS is 0. This change adds the missing entries. Technically only stats_server_set_ts is needed to fix the build error, but from the intent of the code it's clear that stats_pool_set_ts should also be nopped out, despite never being called directly.

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