meson: adjust nlohmann-json dependency

Simplify the dependency detection logic and align the wrap file name
with other repositories.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I93c52a9c4de5fedb47d396ebe12bccbaad701728
diff --git a/meson.build b/meson.build
index a128829..badc8b5 100644
--- a/meson.build
+++ b/meson.build
@@ -82,11 +82,7 @@
 libgpiodcxx = dependency('libgpiodcxx',
     default_options: ['bindings=cxx'])
 
-if cxx.has_header('nlohmann/json.hpp')
-    nlohmann_json_dep = declare_dependency()
-else
-    nlohmann_json_dep = dependency('nlohmann-json')
-endif
+nlohmann_json_dep = dependency('nlohmann_json', include_type: 'system')
 
 if cxx.has_header('CLI/CLI.hpp')
     cli11_dep = declare_dependency()