rhythmdb: Fix build
rhythmdb.h includes librb and libxml but those were not exposed as dependencies in rhythmdb_dep, leading to a build failure on Nix:
FAILED: shell/librhythmbox-core.so.10.0.0.p/rb-history.c.o
[...]
In file included from ../shell/rb-history.h:29,
from ../shell/rb-history.c:33:
../rhythmdb/rhythmdb.h:35:10: fatal error: libxml/tree.h: No such file or directory
35 | #include <libxml/tree.h>
| ^~~~~~~~~~~~~~~