contrib: use pkg-static on all pkg-config files

Use pkg-static to convert all generated .pc files.

That is because every contrib package is statically built,
however when the libring library is built,
pkg-config is not called with the --static flag.

It's done by adding an additional target in main.mak.
pkg-static can be safely run multiple times on the same file.

Change-Id: I2e5c47e793ed5516dd3c8aa0446077bfc945adcb