scripts/combo-layer: fix still overzealous regex in default hook script
In the previous fix to this hook script (OE core revision
e7aae45414e4597e9244f86a81fbc940f73785c8) a start-of-line (^) marker was
missed, so if a commit had no Signed-off-by line but it contained an
inner patch that did, the inner patch was modified causing a "corrupt
patch" error.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>