Work around automake bug (?) to distribute contribs

Files were copied to contrib/src/src instead of contrib/src...
(cherry picked from commit 49399c27)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Makefile.am

@@ -381,7 +381,7 @@ EXTRA_DIST += \
 ##############################################################################
 EXTRA_DIST += \
 	contrib/bootstrap \
-	contrib/src/
+	contrib/src
 
 ###############################################################################
 # Building libvlc