Add missing includes/class declaration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143718 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/DynamicLibrary.h

@@ -17,6 +17,9 @@
 #include <string>
 
 namespace llvm {
+
+class StringRef;
+
 namespace sys {
 
   /// This class provides a portable interface to dynamic libraries which also