Add missing 'dl' dependency to CMakeLists.txt

Fixes the following issue when compiling for me:

Linking CXX executable /home/z3288187/catkin_ws/devel/lib/jaco_driver/jaco_arm_driver
/usr/bin/ld: CMakeFiles/jaco_arm_driver.dir/src/jaco_api.cpp.o: undefined reference to symbol 'dlopen@@GLIBC_2.1'
//lib/i386-linux-gnu/libdl.so.2: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [/home/z3288187/catkin_ws/devel/lib/jaco_driver/jaco_arm_driver] Error 1
make[1]: *** [jaco-ros/jaco_driver/CMakeFiles/jaco_arm_driver.dir/all] Error 2
make: *** [all] Error 2
Invoking "make" failed