Fix base class function call

This was a typo introduced in f6ac5f3d.

Found by looking through NetBSD's GDB patches:
https://github.com/NetBSD/pkgsrc-wip/blob/master/gdb-netbsd/patches/patch-gdb_sparc-nat.h

This patch can't be tested on Linux because Linux does not use the
sparc_target template.

gdb/ChangeLog:

2020-05-23  Christian Biesinger  <cbiesinger@google.com>

        PR gdb/26029:
	* sparc-nat.h (struct sparc_target) <xfer_partial>: Fix base class
	function call.

Change-Id: I4fa88cbdc365efe89b84cc0619b60db38718d9ce
(cherry picked from commit c6a42d11)