oe-git-proxy: Redirect error messages to STDERR

oe-git-proxy: Redirect error messages to STDERR

oe-git-proxy script needs socat. If socat is not found,
an error message is issued on STDOUT. This leads to a misleading
git message:

fatal: protocol error: bad line length character: ERRO

instead of the intended message:

ERROR: socat binary not in PATH

Redirecting the error message to STDERR fixes this issue.

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>