[package] block-mount: fsck.sh should only call pi_include() if dir /lib/functions...

[package] block-mount: fsck.sh should only call pi_include() if dir /lib/functions/fsck exists.

/etc/functions.sh:pi_include() checks if the argument exists and prints
a warning if not. To prevent this warning if package block-mount is installed
but not package e2fsprogs, the script should check if this directory exists
before calling pi_include()

A wrong patch to suppress this warning was previously posted
with subject:
	[PATCH] Fix typo in name of to be included file

Signed-off-by: Mark Vels <mark.vels@team-embedded.nl>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@26503 3c298f89-4303-0410-b956-a3cf2f4a3e73