debug build fail on linux

[simonhong]

ERROR at //brave/build/linux/channels.gni:22:3: Assertion failed.
assert(false, "Not supported channel name: $brave_channel")
^-----
Not supported channel name: development
See //brave/BUILD.gn:17:3: whence it was imported.
import("//brave/build/linux/channels.gni")
^----------------------------------------

In channels.gni, development channel string should considered.
This is regression of #653.