Fix -Wshadow error in ada-valprint.c

Fix -Wshadow error in ada-valprint.c

gdb/ChangeLog:

        * ada-valprint.c (ada_val_print_1): Move the code handling
        TYPE_CODE_ENUM inside its own lexical block.  Declare
        variables len and val there, instead of in the function's
        top level block.  Avoid declaring deref_val again in a way
        that shadows another variable of the same name declared
        in one of the up-level blocks.  Just re-use the up-level
        variable instead.