Fix a build failure.

When I added a use of PRIx32 to one of Pageant's debugging messages a
couple of days ago, I forgot that one of my build setups can't cope
with inclusion of <inttypes.h>, and somehow also forgot the
precautionary pre-push full build that would have reminded me.

defs.h

@@ -17,6 +17,7 @@
 
 #if defined _MSC_VER && _MSC_VER < 1800
 /* Work around lack of inttypes.h in older MSVC */
+#define PRIx32 "x"
 #define PRIu64 "I64u"
 #define SCNu64 "I64u"
 #else