runtime: fix cmallocgc's C prototype
runtime: fix cmallocgc's C prototype

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/121910043

Committer: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go