gcc-plugins: Fix build for upcoming GCC release

The upcoming GCC release has refactored the gimple plugin interface a
bit and unless gimple-iterator.h is included before gimple-fold.h I end
up with a bunch of missing declarations when building the stack
protector plugin.

Link: https://inbox.sourceware.org/gcc-patches/CAFiYyc2q%2Bc-0uZb-zAskmR_U%2BM%2BxQfp-W00ZbLErfjx=j5qHGg@mail.gmail.com/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

scripts/gcc-plugins/gcc-common.h

@@ -72,6 +72,7 @@
 #include "stor-layout.h"
 #include "internal-fn.h"
 #include "gimple-expr.h"
+#include "gimple-iterator.h"
 #include "gimple-fold.h"
 #include "context.h"
 #include "tree-ssa-alias.h"
@@ -88,7 +89,6 @@
 #include "gimple.h"
 #include "tree-phinodes.h"
 #include "tree-cfg.h"
-#include "gimple-iterator.h"
 #include "gimple-ssa.h"
 #include "ssa-iterators.h"