Fix hash_for undefined reference in hipTestConstant test

Issue: mismatch undefined symbols in different user env
  - Binary expects modified return value std::string&
  - Fails to match libhip_hcc.so: return value is std::string& but doesn't match modified C++ env
Fix: Change return value to char*, create new key std::string in header from char*

src/hip_module.cpp

@@ -287,8 +287,8 @@ namespace hip_impl {
         return hmod->executable;
     }
 
-    const std::string& hash_for(hipModule_t hmod) {
-        return hmod->hash;
+    const char* hash_for(hipModule_t hmod) {
+        return hmod->hash.c_str();
     }
 
     hsa_agent_t this_agent() {