package/quota: add optional support for e2fsprogs

When e2fsprogs was compiled before, quota will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/sbin/quotacheck | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [libcom_err.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libext2fs.so.2]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]

Quote from configure log before this patch when e2fsprogs were missing:

checking ext2fs/ext2fs.h usability... no
checking ext2fs/ext2fs.h presence... no
checking for ext2fs/ext2fs.h... no
checking for com_err in -lcom_err... no
checking for ext2fs_initialize in -lext2fs... no
configure: WARNING: Ext2 direct support won't be compiled. Libraries not found.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>