bpftool: Don't try to return value from void function in skeleton

A skeleton generated by bpftool previously contained a return followed
by an expression in OBJ_NAME__detach(), which has return type void. This
did not hurt, the bpf_object__detach_skeleton() called there returns
void itself anyway, but led to a warning when compiling with e.g.
-pedantic.

Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220726133203.514087-1-jthinz@mailbox.tu-berlin.de

tools/bpf/bpftool/gen.c

@@ -1175,7 +1175,7 @@ static int do_skeleton(int argc, char **argv)
 		static inline void					    \n\
 		%1$s__detach(struct %1$s *obj)				    \n\
 		{							    \n\
-			return bpf_object__detach_skeleton(obj->skeleton);  \n\
+			bpf_object__detach_skeleton(obj->skeleton);	    \n\
 		}							    \n\
 		",
 		obj_name