Fix building with --disable-doh

Fix building with --disable-doh

Commit b69e783164cd50e3306364668558e460617ee8fc inadvertently caused
builds using the --disable-doh switch to fail, by putting the
declaration of the isc__nm_async_settlsctx() function inside an #ifdef
block that is only evaluated when DNS-over-HTTPS support is enabled.
This results in the following compilation errors being triggered:

    netmgr/netmgr.c:2657:1: error: no previous prototype for 'isc__nm_async_settlsctx' [-Werror=missing-prototypes]
     2657 | isc__nm_async_settlsctx(isc__networker_t *worker, isc__netievent_t *ev0) {
          | ^~~~~~~~~~~~~~~~~~~~~~~

Fix by making the declaration of the isc__nm_async_settlsctx() function
in lib/isc/netmgr/netmgr-int.h visible regardless of whether
DNS-over-HTTPS support is enabled or not.