connman: Add __USE_GNU to CFLAGS
connman: Add __USE_GNU to CFLAGS

We need to define this when on musl otherwise we end up with

gdhcp/common.h:175:8: error: redefinition of 'struct in6_pktinfo'

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

armcc/meta-musl,kraj/meta-musl