gn2bp: have //net:net export quiche proto headers

Public header in //net:net includes quiche proto headers, without making
the dependency public (or there is an issue in gn desc generation, I am
not sure).

Test: //components/cronet/android:cronet
Change-Id: I1cb0ed0d8c194b8d1c563509d57f0d2c2f731bfa