hdb: add missing build dependency on "hdb-protos.h"

Noticed missing target directory dependency as a build failure in
`make --shuffle` mode (added in https://savannah.gnu.org/bugs/index.php?62100):

    make[2]: Leaving directory '/build/heimdal/lib/gss_preauth'
    Making all in hdb
    make[2]: Entering directory '/build/heimdal/lib/hdb'
    ../../lib/asn1/asn1_compile --option-file=./hdb.opt ./hdb.asn1 hdb_asn1
    for genfile in 'asn1_Event.c asn1_GENERATION.c asn1_HDB_EncTypeList.c asn1_HDB_Ext_Aliases.c asn1_HDB_Ext_Constrained_delegation_acl.c asn1_HDB_Ext_KeyRotation.c asn1_HDB_Ext_KeySet.c asn1_HDB_Ext_Lan_Manager_OWF.c asn1_HDB_Ext_Password.c asn1_HDB_Ext_PKINIT_acl.c asn1_HDB_Ext_PKINIT_cert.c asn1_HDB_Ext_PKINIT_hash.c asn1_HDB_EntryOrAlias.c asn1_HDB_entry_alias.c asn1_HDB_entry.c asn1_HDB_extension.c asn1_HDB_extensions.c asn1_HDB_keyset.c asn1_HDBFlags.c asn1_Key.c asn1_KeyRotation.c asn1_KeyRotationFlags.c asn1_Keys.c asn1_Salt.c'; do \
            true -style='{BasedOnStyle: Mozilla, AlwaysBreakAfterReturnType: TopLevelDefinitions, IndentWidth: 4, SortIncludes: false}' -i ${genfile}; \
    done
    ../../lib/com_err/compile_et hdb_err.et
    make  all-am
    make[3]: Entering directory '/build/heimdal/lib/hdb'
      CC       hdb-ldap.lo
    In file included from hdb_locl.h:67,
                     from hdb-ldap.c:36:
    ./hdb.h:337:10: fatal error: hdb-protos.h: No such file or directory
      337 | #include <hdb-protos.h>
          |          ^~~~~~~~~~~~~~
    compilation terminated.

The change moves hdb-protos.ha and hdb-private.h to BUILT_SOURCES
to guarantee their presence when main build starts.

lib/hdb/Makefile.am

@@ -17,7 +17,9 @@ endif
 BUILT_SOURCES = \
 	$(gen_files_hdb)	\
 	hdb_err.c \
-	hdb_err.h
+	hdb_err.h \
+	$(srcdir)/hdb-protos.h \
+	$(srcdir)/hdb-private.h
 
 gen_files_hdb = \
 	asn1_Event.c \