gn2bp: properly forward genrule dependency

I checked, for //base:base, all outputs have the same extension (it
would be terrible it this were not the case). Also, //base:base seems
to only generated .h files right now.

Test: //base:base
Change-Id: I2e5e7b51408a072c28fabaf51213c4f0c7b4f7e7
1 file changed