Fix standalone build fail in utils/version.cpp

Either the new compiler, C++ version, or nlohmann version starting
giving an error when building with subprojects:

```
../tools/power-utils/version.cpp: In function ‘PsuVersionInfo utils::getVersionInfo(const std::string&)’:
../tools/power-utils/version.cpp:67:40: error: ambiguous overload for ‘operator=’ (operand types are ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’}
and ‘const nlohmann::basic_json<>::value_type’ {aka ‘const nlohmann::basic_json<>’})
   67 |             versionStr = fru["fileName"];
```

Change-Id: Ic0de80331a04110a279702b30bd7cf55ec8c083c
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/tools/power-utils/version.cpp b/tools/power-utils/version.cpp
index 2becfa5..f24a08e 100644
--- a/tools/power-utils/version.cpp
+++ b/tools/power-utils/version.cpp
@@ -64,7 +64,7 @@
     {
         if (fru["propertyName"] == "Version")
         {
-            versionStr = fru["fileName"];
+            versionStr = fru["fileName"].get<std::string>();
             break;
         }
     }