Fix up style of StaticString declarations.

At the top level of a file, "const" is the same as "static const",
and reduce the stuttering.

hphp/runtime/base/program_functions.cpp

@@ -124,7 +124,7 @@ time_t start_time() {
   return s_startTime.startTime;
 }
 
-static const StaticString
+const StaticString
   s_HPHP("HPHP"),
   s_HHVM("HHVM"),
   s_HHVM_JIT("HHVM_JIT"),