fix detection of static icu under unix

the library has a dependency on libdl.

Task-number: QTBUG-58301
Change-Id: I36567ded32980b241ff2f01cfdec044510405a75
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/corelib/configure.json

@@ -56,6 +56,9 @@
                 },
                 { "libs": "-licuin -licuuc -licudt", "condition": "config.win32 && features.shared" },
                 { "libs": "-licui18n -licuuc -licudata", "condition": "!config.win32" }
+            ],
+            "use": [
+                { "lib": "libdl", "condition": "features.dlopen" }
             ]
         },
         "journald": {