tools/nolibc: fix misleading help message regarding installation path

The help message says the headers are going to be installed into
tools/include/nolibc but this is only the default if $OUTPUT is not set,
so better clarify this (the current value of $OUTPUT is already shown).

Signed-off-by: Willy Tarreau <[e-mail]>
Signed-off-by: Thomas Weißschuh <[e-mail]>

tools/include/nolibc/Makefile

···
 	@echo "Supported targets under nolibc:"
 	@echo "  all                 call \"headers\""
 	@echo "  clean               clean the sysroot"
-	@echo "  headers             prepare a sysroot in tools/include/nolibc/sysroot"
+	@echo "  headers             prepare a sysroot in \$${OUTPUT}sysroot"
 	@echo "  headers_standalone  like \"headers\", and also install kernel headers"
 	@echo "  help                this help"
 	@echo ""