[Ada] move some variables to scope where they are used

[Ada] move some variables to scope where they are used

I noticed that some variables are only used inside one side of
an if/else blob. So I moved these variables inside that block for
better clarity.

gdb/ChangeLog:

        * ada-valprint.c (ada_val_print_array): Move variables `eltlen'
        and `len' declaration and computation inside block where they
        are being used.