Add lz4 to build dependencies

This commit fixes build failure due to failed "make check".

Our test-compress uses lz4cat. Binary is included in lz4 package that
used to contain both library and binaries. Since lz4-libs split-off we
need to explicitly pull-in lz4.

diff --git a/systemd.spec b/systemd.spec
index 9b344f7..3bb3b01 100644
--- a/systemd.spec
+++ b/systemd.spec
@@ -109,6 +109,7 @@
 %if %{num_patches}
 BuildRequires:  git
+BuildRequires:  lz4
 Requires(post): coreutils
 Requires(post): sed