Fix non-pointer type compilation error in aix-thread.c

Fix non-pointer type compilation error in aix-thread.c

In aix-thread.c we use ms->value_address () to get the symbol address.
This triggers the following compiler error...

     base operand of '->'  has non-pointer type 'bound_minimal_symbol'

... because ms is not a pointer.

This commit fixes this error by using ms.value_address () instead.