build: Add a Libs.private field to overlay pkg-config file

We need to list all internal libraries there so that we can statically
link.

Fixes: commit 2374f409

src/libbsd-overlay.pc.in

@@ -8,4 +8,5 @@ Description: Utility functions from BSD systems (overlay)
 Version: @VERSION@
 URL: https://libbsd.freedesktop.org/
 Libs: -L${libdir} -lbsd
+Libs.private: @LIBBSD_LIBS@
 Cflags: -isystem ${includedir}/bsd -DLIBBSD_OVERLAY