spec: Remove deleted libbtf.h from the RPM file list

It should have been removed from the specfile when it was deleted from
the source tree, fix it.

Fixes: 743f2536d8b876a4 ("btf_encoder: Move libbtf.c to btf_encoder.c, the only user of its functions")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
1 file changed