libm: add braces around initialization of subobjects

This cleans up a warning when building libm at higher WARNS levels and
makes the intent more clear. By the C standard the values are assigned
to subobject members in order so this change introduces no functional
change. (6.7.9 20)

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8333