Update nlohmann-json subproject handling

Without this, a build with subprojects would fail with:

```
meson.build:89:24: ERROR: Include dir subprojects/nlohmann-json/single_include does not exist.
```

Change-Id: I26e26a7a0473ffb163fc4ca9f1ac291f13de261e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/meson.build b/meson.build
index 369d256..b9b2d87 100644
--- a/meson.build
+++ b/meson.build
@@ -85,13 +85,7 @@
 if cxx.has_header('nlohmann/json.hpp')
     nlohmann_json_dep = declare_dependency()
 else
-    subproject('nlohmann', required: false)
-    nlohmann_json_dep = declare_dependency(
-        include_directories: [
-            'subprojects/nlohmann-json/single_include',
-            'subprojects/nlohmann-json/single_include/nlohmann',
-        ]
-    )
+    nlohmann_json_dep = dependency('nlohmann-json')
 endif
 
 if cxx.has_header('CLI/CLI.hpp')