Don't link with -lrt

Index: main/CMakeLists.txt
--- main/CMakeLists.txt.orig
+++ main/CMakeLists.txt
@@ -251,7 +251,7 @@ else (MINGW)
       if (APPLE)
         target_link_libraries(mscore ${OsxFrameworks})
       else (APPLE)
-          target_link_libraries(mscore rt)
+          target_link_libraries(mscore)
       endif (APPLE)
 
       if (APPLE)
