blemesh: Fix compiler error when building without PB_ADV
blemesh: Fix compiler error when building without PB_ADV

Error: repos/apache-mynewt-core/net/nimble/host/mesh/src/prov.c: In
function 'bt_mesh_prov_reset_link':
repos/apache-mynewt-core/net/nimble/host/mesh/src/prov.c:1590:9:
error:
    'struct prov_link' has no member named 'rx'
         link.rx.buf = bt_mesh_proxy_get_buf();

IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,IMGJulian/incubator-mynewt-core