gcc-source: fix bug to avoid building error.

gcc-source: fix bug to avoid building error.

gcc-source don't do do_package_write_rpm, so we should set
PACKAGES = "" to avoid the building error if we want generate srpm,
otherwise, we get the error as below.

ERROR: Task do_deploy_archives in meta/recipes-devtools/gcc/gcc-source_4.9.bb \
       depends upon non-existent task do_package_write_rpm in \
       meta/recipes-devtools/gcc/gcc-source_4.9.bb

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>