Fix warning on PowerPC

GCC 5+ will generate "error: comparison is always true due to limited range of data type" when -Wextra is used because PowerPC by default uses unsigned char, so it can never be less than 0.
diff --git a/tinyxml2.cpp b/tinyxml2.cpp
index c872a54..7df8443 100755
--- a/tinyxml2.cpp
+++ b/tinyxml2.cpp
@@ -2183,7 +2183,8 @@
     }

     for( int i=0; i<NUM_ENTITIES; ++i ) {

         const char entityValue = entities[i].value;

-        TIXMLASSERT( 0 <= entityValue && entityValue < ENTITY_RANGE );

+        // cast to explicit signed because char may be unsigned (on PowerPC)

+        TIXMLASSERT( 0 <= static_cast<signed char>(entityValue) && entityValue < ENTITY_RANGE );

         _entityFlag[ (unsigned char)entityValue ] = true;

     }

     _restrictedEntityFlag[(unsigned char)'&'] = true;