dist: fix build error with debuginfo package

Fixes following error:

+ '%{__debug_install_post}'
/var/tmp/rpm-tmp.me90D2: line 86: fg: no job control

Signed-off-by: Kairui Song <kasong@tencent.com>

dist/templates/kernel.template.spec

@@ -1134,6 +1134,7 @@ ln -sf %{debuginfo_dir}/lib/modules/$KernUnameR/vmlinux %{buildroot}/boot/vmlinu
 rm -rf %{buildroot}%{debuginfo_dir}/usr/src
 
 %undefine __debug_install_post
+%global __debug_install_post %{nil}
 
 ###### Finally, module sign and compress ######
 %if %{with_modsign} && %{with_core}