drd/Makefile.am: Add -Wswitch-enum

Hopefully one day this compiler flag can be enabled for the entire Valgrind
code base.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14548 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/drd/Makefile.am b/drd/Makefile.am
index 3b0771b..9ba4c58 100644
--- a/drd/Makefile.am
+++ b/drd/Makefile.am
@@ -38,6 +38,7 @@
   -O2					\
   @FLAG_W_EXTRA@			\
   -Wformat-nonliteral			\
+  -Wswitch-enum			        \
   -Wno-inline				\
   -Wno-unused-parameter