gdb: don't include gdbsupport/array-view.h in defs.h

Nothing in defs.h actually uses this.  Everything that I can compile
still compiles, so I guess that all users of array_view already include
it one way or another.  Worst case, if this causes some build failure,
the fix will be one #include away.

Change-Id: I981be98b0653cc18c929d85e9afd8732332efd15