List zlib as another dependency of libbpf in pkg-config template.
Verified it is correctly resolved to proper -lz flag:

$ make DESTDIR=/tmp/libbpf-install install
$ pkg-config --libs /tmp/libbpf-install/usr/local/lib64/pkgconfig/libbpf.pc
-L/usr/local/lib64 -lbpf
$ pkg-config --libs --static /tmp/libbpf-install/usr/local/lib64/pkgconfig/libbpf.pc
-L/usr/local/lib64 -lbpf -lelf -lz

Fixes: 166750bc ("libbpf: Support libbpf-provided extern variables")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Cc: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/bpf/20191216183830.3972964-1-andriin@fb.com