Suppress gcc 4.x -Wpointer-sign (included in -Wall) warnings

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5275 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/linux-user/uaccess.c b/linux-user/uaccess.c
index ed50437..4d50693 100644
--- a/linux-user/uaccess.c
+++ b/linux-user/uaccess.c
@@ -62,7 +62,7 @@
         ptr = lock_user(VERIFY_READ, guest_addr, max_len, 1);
         if (!ptr)
             return -TARGET_EFAULT;
-        len = qemu_strnlen(ptr, max_len);
+        len = qemu_strnlen((const char *)ptr, max_len);
         unlock_user(ptr, guest_addr, 0);
         guest_addr += len;
         /* we don't allow wrapping or integer overflow */