Add missing FALLTHROUGH macros in a switch statement

Merged Arаm Sаrgsyаn requested to merge 3313-dlz_ldap_driver.c-fix-fallthrough-v9_16 into v9_16

dzl_ldap_driver.c was missing some FALLTHROUGH macros in a switch statement resulting in compiler warnings and a failed build when configured with --with-dlz-ldap option.

Closes #3313 (closed)

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