make lookupSymbol threadsafe, so loading doesn't block all threads on systems without an IO manager thread (win32)

src/System/Plugins/Load.hs

@@ -675,7 +675,7 @@ getImports m = do
 -- ---------------------------------------------------------------------
 -- C interface
 --
-foreign import ccall unsafe "lookupSymbol"
+foreign import ccall threadsafe "lookupSymbol"
    c_lookupSymbol :: CString -> IO (Ptr a)
 
 foreign import ccall unsafe "loadObj"

src/System/Plugins/LoadTypes.hs

@@ -43,6 +43,7 @@ data Module = Module { path  :: !FilePath
                      , iface :: Iface    -- cache the iface
                      , key   :: Key
                      }
+
 instance Ord Module where
     compare m1 m2 = mname m1 `compare` mname m2