mirrors.bbclass: add new GNU_MIRROR
mirrors.bbclass: add new GNU_MIRROR

Add ftp://mirrors.usc.edu/pub/gnu as a new GNU_MIRROR because the
binutils_2.21 source package could not be found on any of the defined
GNU mirrors.

Signed-off-by: Apelete Seketeli <5513108e721878f07bfb2a30cd534488c7df106c@seketeli.net>

mrchapp/arago-oe-dev,sledz/oe,sledz/oe,sledz/oe,mrchapp/arago-oe-dev,mrchapp/arago-oe-dev,mrchapp/arago-oe-dev,sledz/oe,sledz/oe,mrchapp/arago-oe-dev,sledz/oe,mrchapp/arago-oe-dev,sledz/oe,mrchapp/arago-oe-dev