QtJson: mark String/Latin1String ctors explicit

An implicit conversion from const char* would be quite unfortunate.
Luckily, the code compiles as-is.

Change-Id: I445f983a27cc25bfaf4285c1a6c5811bd5d201b3
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/json/qjson_p.h

@@ -307,7 +307,7 @@ class Latin1String;
 class String
 {
 public:
-    String(const char *data) { d = (Data *)data; }
+    explicit String(const char *data) { d = (Data *)data; }
 
     struct Data {
         qle_int length;
@@ -381,7 +381,7 @@ public:
 class Latin1String
 {
 public:
-    Latin1String(const char *data) { d = (Data *)data; }
+    explicit Latin1String(const char *data) { d = (Data *)data; }
 
     struct Data {
         qle_short length;