add pkg config file

pkgconfig/libcoincellhell.pc.in

+includedir=@CMAKE_INSTALL_PREFIX@/include
+
+Name: libcoincellhell
+Description: C lib to control CoincellHell devices
+Version: 1.0
+Libs: -L${libdir} -lcoincellhell
+Cflags: -I${includedir}