package/go-bootstrap: Add toolchain dependency

To build programs that need cgo support the toolchain needs to
be available.

Cc: Christian Stewart <christian@paral.in>
Signed-off-by: Geoff Levand <geoff@infradead.org>
[Thomas: add comment in the code about the toolchain dependency.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>