claws-mail: add curl dependency

* it's needed by libetpan:
  | configure:20812: checking whether libetpan-config hints compiles and links fine
  | configure:20826: arm-oe-linux-gnueabi-gcc  -march=armv5te -marm -mthumb-interwork --sysroot=sysroots/qemuarm -o conftest -O2 -pipe -g -feliminate-unused-debug-types -Wno-unused-function -Wno-pointer-sign -Wall   -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed conftest.c  -lm  -Lsysroots/qemuarm/usr/lib -letpan -pthread -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -lcurl -lgcrypt -lgnutls -lz -lexpat -lgnutls -llockfile >&5
  | sysroots/x86_64-linux/usr/libexec/armv5te-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/4.8.1/ld: error: cannot find -lcurl
  | collect2: error: ld returned 1 exit status

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