fix build with make 3.81

fix build with make 3.81

make 3.81 doesn't support multiline variables defined with

 define var =
 ...
 endef

Dropping the "=" in the first line will fix the issue.

Fixes: ded08cdfa72bb ("tools: generate most contents of library make variables")
Fixes: ddb2934a914df ("stubdom: add correct dependencies for Xen libraries")
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>