MINOR: quic: Missing QUIC encryption level for qc_build_pkt()

MINOR: quic: Missing QUIC encryption level for qc_build_pkt()

qc_build_pkt() has recently been modified to support any type of
supported frame at any encryption level (assuming that an encryption level does
not support any type of frame) but quic_tls_level_pkt_type()
prevented it from building application level packet type because it was written
only for the handshake.
This patch simply adds the remaining encryption level QUIC_TLS_ENC_LEVEL_APP
which must be supported by quic_tls_level_pkt_type().