monitor: Remove use of nlohmann::json::get_ptr()

A new version of nlohmann JSON changed how it handles get_ptr and may
not return anything if it's asked for a signed value but internally the
value is unsigned.  Instead just use get<>() which doesn't have that
issue.

Tested:
Fan monitor still works, though there is no JSON available right now
that would force a code path that uses this.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I1ba0b66652e4f920d3253827aaa921601c48d1c7
diff --git a/monitor/types.hpp b/monitor/types.hpp
index b5848f5..dceb4da 100644
--- a/monitor/types.hpp
+++ b/monitor/types.hpp
@@ -58,27 +58,27 @@
                                             const std::string& type = "")
     {
         PropertyValue value;
-        if (auto boolPtr = entry.get_ptr<const bool*>())
+        if (entry.is_boolean())
         {
             if (type.empty() || type == "bool")
             {
-                value = *boolPtr;
+                value = entry.get<bool>();
                 return value;
             }
         }
-        if (auto int64Ptr = entry.get_ptr<const int64_t*>())
+        if (entry.is_number_integer())
         {
             if (type.empty() || type == "int64_t")
             {
-                value = *int64Ptr;
+                value = entry.get<int64_t>();
                 return value;
             }
         }
-        if (auto stringPtr = entry.get_ptr<const std::string*>())
+        if (entry.is_string())
         {
             if (type.empty() || type == "std::string")
             {
-                value = *stringPtr;
+                value = entry.get<std::string>();
                 return value;
             }
         }