dns_over_https/main includes time_sync.h, nvs_flash.h, esp_event.h,
esp_timer.h, and mbedtls headers. Declare nvs_flash, esp_event,
esp_timer, mbedtls and time_sync in PRIV_REQUIRES so the build is
self-describing under any build system.
Also move esp_netif from PRIV_REQUIRES to REQUIRES in
protocol_examples_common, since its public header exposes esp_netif.h.
Also add the PRIVATE keyword to target_link_libraries in the
dns_over_https component — CMake rejects mixed keyword/plain
signatures on the same target.
