meson: adjust nlohmann-json dependency

Simplify nlohmann-json dependency logic and align wrap file name with
other repositories.

Change-Id: Ie8a3590a474c0138932230fc9e9f11e457cbabea
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 059f043..72e810b 100644
--- a/meson.build
+++ b/meson.build
@@ -50,6 +50,8 @@
     endif
 endif
 
+nlohmann_json_dep = dependency('nlohmann_json', include_type: 'system')
+
 phosphor_dbus_interfaces = dependency('phosphor-dbus-interfaces',
     default_options: [ 'data_com_ibm=true', 'data_org_open_power=true' ],
     fallback: ['phosphor-dbus-interfaces', 'phosphor_dbus_interfaces_dep'])
@@ -81,12 +83,6 @@
     CLI11_dep = dependency('CLI11')
 endif
 
-if compiler.has_header('nlohmann/json.hpp')
-    nlohmann_json_dep = declare_dependency()
-else
-    nlohmann_json_dep = dependency('nlohmann-json')
-endif
-
 if not build_tests.disabled()
     subdir('test')
 endif