ntbtls.pc: Fix library dependency.

* src/ntbtls.pc.in (Requires): Remove libgcrypt and ksba.
(Requires.private): New with libgcrypt and ksba.
(Libs.private: New with forgotten @NETLIB@.

libgcrypt and ksba are only needed for static linking, because it's
used internally.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>