make: Always set LC_ALL=C for makeinfo

Otherwise the generated file qemu-doc.html will contain "Anhang"
instead of "Appendix" with a German locale (de_DE.UTF-8).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>