make PCH depend on LinkTarget/headers
make PCH depend on LinkTarget/headers

Rather than LinkTarget/external_headers, as this doesn't really make
any difference, except if PCH is made to also use headers from the same
target (not a very good idea for development, but might be useful
e.g. for buildbots).

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core