Fix base class function call

This was a typo introduced in f6ac5f3d63.

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:

2020-02-05  Christian Biesinger  <cbiesinger@google.com>

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

Change-Id: I4fa88cbdc365efe89b84cc0619b60db38718d9ce

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-05  Christian Biesinger  <cbiesinger@google.com>
+
+	* sparc-nat.h (struct sparc_target) <xfer_partial>: Fix base class
+	function call.
+
 2020-02-05  Christian Biesinger  <cbiesinger@google.com>
 
 	* ppc-nbsd-tdep.h: Fix macro name in #endif comment.

gdb/sparc-nat.h

@@ -75,8 +75,8 @@ struct sparc_target : public BaseTarget
       return sparc_xfer_wcookie (object, annex, readbuf, writebuf,
 				 offset, len, xfered_len);
 
-    return BaseTarget (object, annex, readbuf, writebuf,
-		       offset, len, xfered_len);
+    return BaseTarget::xfer_partial (object, annex, readbuf, writebuf,
+				     offset, len, xfered_len);
   }
 };