[meson] Fix 'iwyu' target for meson >= 0.57

The run_target() function requires an absolute path in meson >= 0.57.

meson.build

@@ -14,8 +14,10 @@ emper_dependencies = [thread_dep, uring_dep]
 
 boost_thread_dep = dependency('boost', modules : ['thread'], required: false)
 
+tools_dir = join_paths(meson.source_root(), 'tools')
+
 run_target('iwyu',
-		   command: 'tools/check-iwyu')
+		   command: join_paths(tools_dir, 'check-iwyu'))
 
 conf_data = configuration_data()
 option_urcu = get_option('userspace_rcu')