Improve out-of-range check in gimp_backtrace_find_thread_by_id().

Remove unnecessary #include <exchndl.h> in gimpbacktrace-windows.c,
and revert commit 644234e9 (the
DrMingw detection happens at runtime).  The Windows backend can
work without DrMingw, it just can't find all the symbols, and
doesn't provide source-location information.