scripts/combo-layer: fix overzealous regex in default hook script

scripts/combo-layer: fix overzealous regex in default hook script

combo-layer's hook scripts are intended to modify patches as they pass
through; the default one adds a prefix with the component name and
a line with the component repo revision before the Signed-off-by;
however the script was also unintentionally modifying the contents of
patches *within* the patches passing through it, which resulted in
unexpected failures when the combo-layer script attempted to use
"git am" to apply them.

(From OE-Core rev: e7aae45414e4597e9244f86a81fbc940f73785c8)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>