gdb/ada-exp.y: Remap yydefred

On:

 $ uname -a
 NetBSD gcc70.fsffrance.org 5.1 NetBSD 5.1 (GENERIC) #0: Sat Nov  6 13:19:33 UTC 2010  builds@b6.netbsd.org:/home/builds/ab/netbsd-5-1-RELEASE/amd64/201011061943Z-obj/home/builds/ab/netbsd-5-1-RELEASE/src/sys/arch/amd64/compile/GENERIC amd64

The link fails with:

 (...)
 d-exp.o: In function `parse_number':
 ../../src/gdb/d-exp.y:762: multiple definition of `yydefred'
 ada-exp.o:/home/palves/gdb/build/gdb/ada-lex.c:925: first defined here
 ld: Warning: size of symbol `yydefred' changed from 464 in ada-exp.o to 336 in d-exp.o
 Makefile:1404: recipe for target 'gdb' failed

NetBSD's yacc uses a "yydefred" symbol that we missed renaming in the
Ada parser.  All other gdb parsers do this already.

gdb/ChangeLog:
2016-04-16  Pedro Alves  <palves@redhat.com>

	* ada-exp.y (yydefred): Define as ada_yydefred.