[TW] Core/Spell: SPELL_AURA_OBS_MOD_HEALTH

- Add back line that was moved in
https://github.com/TrueWoW/TrinityCore/commit/6c25f44da0f130e06284c1bd778629082a7afa46
needed for area auras such as spirit bond
Merge pull request #65 from gecko32/Patch8

[TW] Core/Spell: SPELL_AURA_OBS_MOD_HEALTH