[NFC][OCaml] Fix implicit declaration compilation warnings

[jberdine]

Summary

2dd4f35c7fc75639920ebc473a4a57ea91864c10 moved LLVMAddLowerSwitchPass
and LLVMAddPromoteMemoryToRegisterPass declarations from
llvm-c/Transforms/Scalar.h to llvm-c/Transforms/Utils.h

llvm/trunk/bindings/ocaml/transforms/scalar_opts/scalar_opts_ocaml.c

 |* are by and large transparent wrappers to the corresponding C functions.    *|
 |*                                                                            *|
 |* Note that these functions intentionally take liberties with the CAMLparamX *|
 |* macros, since most of the parameters are not GC heap objects.              *|
 |*                                                                            *|
 \*===----------------------------------------------------------------------===*/
 
 #include "llvm-c/Transforms/Scalar.h"
+#include "llvm-c/Transforms/Utils.h"
 #include "caml/mlvalues.h"
 #include "caml/misc.h"
 
 /* [<Llvm.PassManager.any] Llvm.PassManager.t -> unit */
 CAMLprim value llvm_add_aggressive_dce(LLVMPassManagerRef PM) {
   LLVMAddAggressiveDCEPass(PM);
   return Val_unit;
 }