init: fix build with older gcc

__setup_str is used on arrays of char, so there aren't any relocatable
items. Hence __initconst has to be used here, not __initconstrel.

Whatever led to the revert of commit 59b151d2c0 (the original version
of a6066af5b1 "xen/init: Annotate all command line parameter
infrastructure as const" must have got addressed meanwhile - with the
patch here I can't see that old gcc (4.3ish) report a section type
conflict anymore.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/xen/init.h
+++ b/xen/include/xen/init.h
@@ -88,7 +88,7 @@ struct kernel_param {
 
 extern const struct kernel_param __setup_start[], __setup_end[];
 
-#define __setup_str static const  __initconstrel \
+#define __setup_str static const __initconst \
     __attribute__((__aligned__(1))) char
 #define __kparam static const __initsetup \
     __attribute__((__aligned__(sizeof(void *)))) struct kernel_param