secp256k1: fix 'make dist'

If --enable-libsecp256k1 is not selected, its configure is not run but it is
still considered to be a dist target. 'make dist' fails because no makefile has
been generated. By removing it from DIST_SUBDIRS, 'make dist' would work, but
the resulting tarball would depend on the given configure option, which is not
acceptable.

Instead, treat it like leveldb.

This can be reverted to the previous usag if/when --enable-libsecp256k1 ever
becomes the default, since always configuring the subdir would be reasonable at
that point.