gdb: Fix ATTRIBUTE_NONNULL usage

gdb: Fix ATTRIBUTE_NONNULL usage

Should fix the build failure with Clang mentioned at
	<https://sourceware.org/bugzilla/show_bug.cgi?id=21206#c2>:

 In file included from ../../binutils-gdb/gdb/dwarf2read.c:72:
 ../../binutils-gdb/gdb/common/gdb_unlinker.h:35:35: error: '__nonnull__' attribute is invalid for the implicit this argument
   unlinker (const char *filename) ATTRIBUTE_NONNULL (1)
				   ^                  ~
 ../../binutils-gdb/gdb/../include/ansidecl.h:169:48: note: expanded from macro 'ATTRIBUTE_NONNULL'
 #  define ATTRIBUTE_NONNULL(m) __attribute__ ((__nonnull__ (m)))

gdb/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR 21206
	* common/gdb_unlinker.h (unlinker::unlinker): Attribute nonnull
	goes to argument 2, not 1.