BUILD: quic: Wrong HKDF label constant variable initializations

BUILD: quic: Wrong HKDF label constant variable initializations

Non constant expressions were used to initialize constant variables leading to
such compilation errors:
	src/xprt_quic.c:66:3: error: initializer element is not a constant expression
   .key_label_len    = strlen(QUIC_HKDF_KEY_LABEL_V1),
Reproduced with CC=gcc-4.9 compilation option.
Fix using macros for each HKDF label.