meson: wrap: Track upstream HEAD

This is what other OpenBMC projects do.  Tracking HEAD can result in
reproducibility or strange ordering issues caused by upstream activity;
however, it does provide some benefits such as quickly identifying
upstream bugs or finding incompatible API changes sooner rather than
later.  Given that the meson subproject workflow is a -secondary-
workflow typically used by developers, these are valuable trade-offs and
the usual stability contract need not be guaranteed.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Change-Id: Iec7dfe4244b2dfb620fc18cedca3a9615bc275e7
diff --git a/subprojects/nlohmann.wrap b/subprojects/nlohmann.wrap
index 3c028c3..dbdadd5 100644
--- a/subprojects/nlohmann.wrap
+++ b/subprojects/nlohmann.wrap
@@ -1,3 +1,3 @@
 [wrap-git]
-revision = e7b3b40b5a95bc74b9a7f662830a27c49ffc01b4
+revision = HEAD
 url = https://github.com/nlohmann/json.git