Makefile: Do not check for libelf when building OOT module

Makefile: Do not check for libelf when building OOT module

When building out-of-tree modules, the necessary tools should have
already been built.  We therefore do not need libelf-dev to be
installed.

This effectively reverts commit 9f0c18aec620 "objtool: Fix
CONFIG_STACK_VALIDATION=y warning for out-of-tree modules", and
similarly moves the check introduced by commit 33a57ce0a54d "bpf:
Compile resolve_btfids tool at kernel compilation start".

Gbp-Pq: Topic debian
Gbp-Pq: Name makefile-do-not-check-for-libelf-when-building-oot-module.patch