meson: add proper cereal dependency

There are source files that assume cereal header files will be present
without the meson build explicitly finding that dependency.  This leads
to compile failures when using the meson-subproject based build.  Add a
proper dependency and wrap file for this, based on similar code found in
phosphor-logging.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6819be89101ded9f8a4dbe885074d30cb06e2df7
diff --git a/subprojects/cereal.wrap b/subprojects/cereal.wrap
new file mode 100644
index 0000000..a4cb7ec
--- /dev/null
+++ b/subprojects/cereal.wrap
@@ -0,0 +1,4 @@
+[wrap-git]
+url = https://github.com/USCiLab/cereal.git
+revision = HEAD
+# need at least for C++20 fixes 3e4d1b84cab4891368d2179a61a7ba06a5693e7f