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>

Makefile

@@ -328,7 +328,7 @@ TEXIFLAG=$(if $(V),,--quiet)
 	$(call quiet-command,texi2dvi $(TEXIFLAG) -I . $<,"  GEN   $@")
 
 %.html: %.texi
-	$(call quiet-command,$(MAKEINFO) $(MAKEINFOFLAGS) --html $< -o $@, \
+	$(call quiet-command,LC_ALL=C $(MAKEINFO) $(MAKEINFOFLAGS) --html $< -o $@, \
 	"  GEN   $@")
 
 %.info: %.texi