libetpan: add liblockfile dependency

* claws-mail failed with
  | configure:20783: checking whether libetpan-config hints compiles and links fine
  | configure:20797: arm-oe-linux-gnueabi-gcc  -march=armv5te  -marm -mthumb-interwork   --sysroot=/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm -o conftest -O2 -pipe -g -feliminate-unused-debug-types -Wno-unused-function -Wno-pointer-sign -Wall -pthread -I/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm/usr/include/enchant -I/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm/usr/include/glib-2.0 -I/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm/usr/lib/glib-2.0/include     -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed conftest.c  -lm  -L/home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/qemuarm/usr/lib -letpan -pthread -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -lgnutls -llockfile -lsasl2 >&5
  | /home/jenkins/oe/shr-core-branches/shr-core/tmp-eglibc/sysroots/x86_64-linux/usr/libexec/armv5te-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/4.7.2/ld: cannot find -llockfile
  | collect2: error: ld returned 1 exit status

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>