gdb: make interp_add static

Since this commit:

  commit 8322445e
  Date:   Tue Jun 21 01:11:45 2016 +0100

      Introduce interpreter factories

Interpreters should be registered with GDB, not by calling interp_add,
but with a call to interp_factory_register.  I've checked the insight
source, and it too has moved over to using interp_factory_register.

In this commit I make interp_add static within interps.c.

There should be no user visible change after this commit.