Fix unused variable warning in qrandom.cpp

The 'ret' variable is unused if assertions are disabled.

Change-Id: I90fec4e18e82d13b681068e222e994247988cbc5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qrandom.cpp

@@ -139,11 +139,13 @@ public:
             // getentropy can't fail under normal circumstances
             int ret = getentropy(reinterpret_cast<uchar *>(buffer) + read, 256);
             Q_ASSERT(ret == 0);
+            Q_UNUSED(ret);
             read += 256;
         }
 
         int ret = getentropy(reinterpret_cast<uchar *>(buffer) + read, count - read);
         Q_ASSERT(ret == 0);
+        Q_UNUSED(ret);
         return count;
     }
 };