Fix gdb build with --enable-build-with-cxx --disable-nls

Compiling gdb with --enable-build-with-cxx --disable-nls, we get:

 .../src/gdb/ada-lang.c:7657:16: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
	type_str = (type != NULL
		 ^
 In file included from .../src/gdb/common/common-defs.h:67:0,
		  from .../src/gdb/defs.h:28,
		  from .../src/gdb/ada-lang.c:21:
 .../src/gdb/common/gdb_locale.h:40:27: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
  # define _(String) (String)
			    ^
 .../src/gdb/ada-lang.c:7730:46: note: in expansion of macro ‘_’
	char *name_str = name != NULL ? name : _("<null>");
					       ^
 Makefile:1140: recipe for target 'ada-lang.o' failed

gdb/ChangeLog:
2016-04-15  Pedro Alves  <palves@redhat.com>

	* ada-lang.c (ada_lookup_struct_elt_type): Constify 'type_str' and
	'name_str' locals.