meson: subproject as system dependency

When phosphor-logging is a subproject, use -isystem for phosphor-logging
headers. This is a signal to, for example static analysis tools, that
the phosphor-logging headers are to be treated as if they were installed
in the compilers default search path (e.g. /usr/include).

Change-Id: I428d507cbf303816b27eae63d1d90d3ff02c2e7d
Signed-off-by: Brad Bishop <bradbish@qti.qualcomm.com>
diff --git a/lib/meson.build b/lib/meson.build
index 2679548..3bd2db0 100644
--- a/lib/meson.build
+++ b/lib/meson.build
@@ -56,3 +56,7 @@
     sources: phosphor_logging_gen,
     dependencies: phosphor_logging_deps,
 )
+
+if meson.is_subproject()
+    phosphor_logging_dep = phosphor_logging_dep.as_system()
+endif