libbpf: fix Makefile exit code if libelf not found

/bin/sh's exit does not recognize -1 as a number, leading to
the following error message:

/bin/sh: 1: exit: Illegal number: -1

Use 1 as the exit code.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>