build: add wrapfile for nlohmann-json

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I1d2c67b7a9db8e0e307d0b5d9efba1ae17c01de3
diff --git a/subprojects/nlohmann-json.wrap b/subprojects/nlohmann-json.wrap
new file mode 100644
index 0000000..9096612
--- /dev/null
+++ b/subprojects/nlohmann-json.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/nlohmann/json.git
+revision = HEAD