fix #2448: compile ISO8859-1 NLS as built-in

Allows to mount VFAT devices even if the currently running kernel was
removed before any VFAT, or other FS using the default Native
Language Support module was mounted during the current uptime.

This then could break updating the ESP partitions, which are mounted
with VFAT in a postrm triggered step - so at a time where the current
/lib/modules/... was already removed, and so the NLS could not get
loaded.

While there are a lot of different NLS, our kernel config has:
> CONFIG_FAT_DEFAULT_IOCHARSET="iso8859-1"

So compile that module as built-in.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

debian/rules

@@ -37,6 +37,7 @@ PVE_CONFIG_OPTS= \
 -e CONFIG_BLK_DEV_SR \
 -e CONFIG_BLK_DEV_DM \
 -e CONFIG_BLK_DEV_NVME \
+-e CONFIG_NLS_ISO8859_1 \
 -d CONFIG_INPUT_EVBUG \
 -d CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND \
 -e CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE \