Fix declaration of sparc_xfer_wcookie

Fix declaration of sparc_xfer_wcookie

When building sparc-nat.c with -Wmissing-declarations, we get:

      CXX    sparc-nat.o
    /home/smarchi/src/binutils-gdb/gdb/sparc-nat.c: In function ‘target_xfer_status sparc_xfer_wcookie(target_ops*, target_object, const char*, gdb_byte*, const gdb_byte*, ULONGEST, ULONGEST, ULONGEST*)’:
    /home/smarchi/src/binutils-gdb/gdb/sparc-nat.c:255:1: error: no previous declaration for ‘target_xfer_status sparc_xfer_wcookie(target_ops*, target_object, const char*, gdb_byte*, const gdb_byte*, ULONGEST, ULONGEST, ULONGEST*)’ [-Werror=missing-declarations]
     sparc_xfer_wcookie (struct target_ops *ops, enum target_object object,
     ^~~~~~~~~~~~~~~~~~

Indeed, the declaration is not in sync with the definition, fix that.

sparc_xfer_wcookie is used in sparc_target::xfer_partial.  sparc_target
is only used in the BSD sparc native files.  The error above was
obtained by running "make sparc-nat.o" on Linux with a cross-compiler
for sparc64-linux-gnu.  But I presume that if we were to build for real
with a BSD/sparc compiler, we would end up with an undefined symbol.

gdb/ChangeLog:

	* sparc-nat.c (sparc_xfer_wcookie): Sync declaration with
	definition.

Change-Id: Id41e706e5516968ff6a49469ddc48eceb29dd3ea