kbuild/mkspec: avoid using brace expansion

Brace expansion might not work properly if _buildshell RPM macro
points to a shell other than bash. Particularly, with _bulidshell
defined to /bin/dash it leads to broken build and source symlinks.

Signed-off-by: Anton Tikhomirov <anton.tikhomirov@cdnetworks.com>
Signed-off-by: Michal Marek <mmarek@suse.com>