spl: x86: Correct the binman symbols for SPL

These symbols are incorrect, meaning that binman cannot find the
associated entry. This leads to errors like:

binman: Section '/binman/simple-bin': Symbol '_binman_spl_prop_size'
   in entry '/binman/simple-bin/u-boot-spl/u-boot-spl-nodtb':
   Entry 'spl' not found in list (mkimage,u-boot-spl-nodtb,
   u-boot-spl-bss-pad,u-boot-spl-dtb,u-boot-spl,u-boot-img,main-section)

Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/spl.h b/include/spl.h
index bb92bc6..8ceb3c0 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -269,8 +269,8 @@
  */
 binman_sym_extern(ulong, u_boot_any, image_pos);
 binman_sym_extern(ulong, u_boot_any, size);
-binman_sym_extern(ulong, spl, image_pos);
-binman_sym_extern(ulong, spl, size);
+binman_sym_extern(ulong, u_boot_spl, image_pos);
+binman_sym_extern(ulong, u_boot_spl, size);
 
 /**
  * spl_get_image_pos() - get the image position of the next phase