[libcxx][AIX] Explicitly include localedef.h in locale.cpp

This header was transitively included to provide the definition of
__lc_ctype_ptr that we use on AIX, but that is fragile as it depends on
the settings of compatibility macros, so we explicitly include it here
to avoid that scenario.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D111239