build: fix handling of dependancies to fix build on openSUSE

build: fix handling of dependancies to fix build on openSUSE

Among other things, openSUSE places seccomp.h inside a non-default
include directory (/usr/include/seccomp/seccomp.h) which revealed
several issues with how dependencies were being handled previously.

The most notable issue is that the include cflags of our build
dependencies were not being provided to the recipes for static
executables (yet they still expected access to the dependency headers).

This also involved a minor cleanup of how these dependencies are
collected, and added liburing to the set of private pkg-config libs
(which I assume was an oversight?).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>