Use relative file paths in .pot file

Otherwise it keeps changing depending on where your working copy
is located, causing unnecessary diffs.
diff --git a/po/CMakeLists.txt b/po/CMakeLists.txt
index 6242b7d..1fd62c2 100644
--- a/po/CMakeLists.txt
+++ b/po/CMakeLists.txt
@@ -12,6 +12,7 @@
 if (GETTEXT_XGETTEXT_EXECUTABLE)
   # Get list of all source files
   file(GLOB_RECURSE po_source
+    RELATIVE ${PROJECT_SOURCE_DIR}
     ${PROJECT_SOURCE_DIR}/vncviewer/*.h
     ${PROJECT_SOURCE_DIR}/vncviewer/*.cxx
   )