libkmod: kmod_log_null: qualify ctx argument as const

libkmod: kmod_log_null: qualify ctx argument as const

kmod_log_null() does not change ctx (does nothing).

Fix warnings

In file included from libkmod/libkmod-index.c:33:
libkmod/libkmod-index.c: In function ‘index_mm_open’:
libkmod/libkmod-index.c:757:6: warning: passing argument 1 of ‘kmod_log_null’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  757 |  DBG(ctx, "file=%s\n", filename);

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>