logger: workaround woefully small BUFSIZ in MUSL

MUSL defines BUFSIZ as 1024 which is not reasonable for log messages.

More broadly, BUFSIZ is the amount of buffering stdio.h does, which
is arbitrary and largely orthogonal to what logging should accept
as the maximum message size.

ASTERISK-29928

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Change-Id: Iaa49fbbab029c64ae3d95e4b18270e0442cce170