blackbox import structs only if global var

The blackbox package now import structs only if the blackboxed contains
global variables, it was importing it everytime before.

 * main.go,
   transform/blackbox.go : Here.
1 job for hm/global in 2 minutes and 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #129590
debian-stable-gcc

00:02:24