x86/boot: don't cast away const-ness

While the general C library strchr() has to do so, the early boot cmdline
parsing has no such requirement.

No functional change.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger@xenproject.org>

xen/arch/x86/boot/cmdline.c

@@ -77,12 +77,12 @@ static int strncmp(const char *cs, const char *ct, size_t count)
     return 0;
 }
 
-static char *strchr(const char *s, int c)
+static const char *strchr(const char *s, int c)
 {
     for ( ; *s != (char)c; ++s )
         if ( *s == '\0' )
             return NULL;
-    return (char *)s;
+    return s;
 }
 
 static size_t strspn(const char *s, const char *accept)