package/gcc: fix list of files of patch file to hash for ccache

package/gcc: fix list of files of patch file to hash for ccache

When we build the list of patches to include to compute the hash for
ccache, a typo precented the special gcc-initial/ and gcc-final/
directories from a global patch dir to be included in the calculation.

Fix that by properly expanding the $(PKG) variable.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: He Chunhui <hchunhui@mail.ustc.edu.cn>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>