dpkg-shlibdeps: don't try to add dependency information for private libraries

dpkg-shlibdeps: don't try to add dependency information for private libraries

Private libraries are defined as libraries without SONAME. They are not listed
in shlibs files and thus there's no point in trying to add a dependency
for them. It also reduce the number of false-positive failures/warnings
generated by dpkg-shlibdeps.