Set json no IO

There's only a couple remaining places we use json IO.  Not including IO
improves compile times in a minor way, because the nlohmann stream/file
template is relatively expensive to compile.

Tested: Launched bmcweb.  Observed bmcweb_persistent_data.json created.
Rebooted bmcweb with 'systemctl restart bmcweb' and observed launched
correctly with persistent file present.

Change-Id: I56f674f20fa8553dc86245a765818849aa5fa102
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/meson.build b/meson.build
index 205ee90..93c26b7 100644
--- a/meson.build
+++ b/meson.build
@@ -197,6 +197,7 @@
             '-DBOOST_URL_NO_SOURCE_LOCATION',
             '-DBOOST_SPIRIT_X3_NO_RTTI',
             '-DJSON_NOEXCEPTION',
+            '-DJSON_NO_IO',
             '-DJSON_USE_IMPLICIT_CONVERSIONS=0',
             '-DOPENSSL_NO_FILENAMES',
             '-DSDBUSPLUS_DISABLE_BOOST_COROUTINES',