Include private dependencies automatically in pkg-config for non-shared builds

When a user invokes pkg-config to get the necessary linker flags
for linking to libhogweed, the user can add --static to get the
private dependencies included, which are necessary for static
linking. If the hogweed build contains both static and shared
libraries, this works as intended - if the user explicitly passes
-static to the linker to have it favor static libs over shared
ones, the same user also needs to tell pkg-config about this intention.

If the hogweed build happens to be static-only, the user of the
library might not be aware of this, and might not realize needing
to pass --static to pkg-config. (This is even more an issue in
setups with a large number of libraries, where only a few of them
are built static-only.)

For these cases, where a library is built as only a static library,
one fairly common convention (not used everywhere, but at least in
some libraries I regularly use) is to include the private dependencies
in the non-private section. This makes sure a user of the library
doesn't need to be concerned about which way this library was built
(unless the user intentionally overrides defaults by passing
flags such as -static to the linker).