PostLink: only stage icu 56 if it exists

otherwise use the installed version.
E.g. at the time of writing latest version 68 works great.

QGCPostLinkCommon.pri

@@ -104,12 +104,17 @@ LinuxBuild {
         libicui18n.so* \
         libQt5TextToSpeech.so.5
 
-    !contains(DEFINES, __rasp_pi2__) {
+    exists($$[QT_INSTALL_LIBS]/libicudata.so.56) {
         # Some Qt distributions link with *.so.56
         QT_LIB_LIST += \
             libicudata.so.56 \
             libicui18n.so.56 \
             libicuuc.so.56
+    } else {
+        QT_LIB_LIST += \
+            libicudata.so \
+            libicui18n.so \
+            libicuuc.so
     }
 
     for(QT_LIB, QT_LIB_LIST) {