ovsdb: Don't add ovsdb-server.c to libovsdb.

Without this change, with shared libraries, VLOG
constructor for ovsdb-server would get called twice corrupting
the 'vlog_modules' list causing an infinite loop.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Reported-by: Gur Stavi <gstavi@mrv.com>
Acked-by: Ben Pfaff <blp@nicira.com>