crypto: make root_cert_store variable extern

Before this commit it was declared static (in a header file!), meaning
it got duplicated in every file that includes it.

A few duplicated pointers is not the end of the world but it introduces
a lot of potential for confusion because root_cert_store in file A is
not the root_cert_store in file B.

Moral of the story: don't declare static variables in header files.