cmd/gc: fix build
cmd/gc: fix build

Darwin 10.6 (gcc 4.2) and some older versions of gcc default to C90 mode, not C99 mode. Silence the warning.

LGTM=aram, iant
R=golang-codereviews, aram, iant
CC=golang-codereviews
https://codereview.appspot.com/83090050

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go