selftests: bpf: Add files generated after build to .gitignore

The following files are generated after building /selftests/bpf/ and
should be added to .gitignore:

	- libbpf.pc
	- libbpf.so.*

Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>