BUG/MINOR: 51d: Aligned const pointers to changes in 51Degrees.

Parameters provided to 51Degrees methods that have changed to require
const pointers are now cast to avoid compiler warnings.

This should be backported to 1.6.
diff --git a/src/51d.c b/src/51d.c
index 0211dd7..0a70d7a 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -269,7 +269,7 @@
 static void _51d_process_match(const struct arg *args, struct sample *smp)
 {
 	char valuesBuffer[1024];
-	char **requiredProperties = fiftyoneDegreesGetRequiredPropertiesNames();
+	const char **requiredProperties = fiftyoneDegreesGetRequiredPropertiesNames();
 	int requiredPropertiesCount = fiftyoneDegreesGetRequiredPropertiesCount();
 	fiftyoneDegreesDeviceOffsets *deviceOffsets = &global._51degrees.device_offsets;
 
@@ -501,7 +501,7 @@
 	global._51degrees.header_offsets = (int32_t*)malloc(global._51degrees.header_count * sizeof(int32_t));
 	for (index = 0; index < global._51degrees.header_count; index++) {
 		global._51degrees.header_offsets[index] = fiftyoneDegreesGetHttpHeaderNameOffset(index);
-		global._51degrees.header_names[index].str = fiftyoneDegreesGetHttpHeaderNamePointer(index);
+		global._51degrees.header_names[index].str = (char*)fiftyoneDegreesGetHttpHeaderNamePointer(index);
 		global._51degrees.header_names[index].len = strlen(global._51degrees.header_names[index].str);
 		global._51degrees.header_names[index].size = global._51degrees.header_names[index].len;
 	}
@@ -531,10 +531,10 @@
 	}
 
 #ifdef FIFTYONEDEGREES_H_PATTERN_INCLUDED
-	_51d_dataset_status = fiftyoneDegreesInitWithPropertyArray(global._51degrees.data_file_path, &global._51degrees.data_set, _51d_property_list, i);
+	_51d_dataset_status = fiftyoneDegreesInitWithPropertyArray(global._51degrees.data_file_path, &global._51degrees.data_set, (const char**)_51d_property_list, i);
 #endif
 #ifdef FIFTYONEDEGREES_H_TRIE_INCLUDED
-	_51d_dataset_status = fiftyoneDegreesInitWithPropertyArray(global._51degrees.data_file_path, _51d_property_list, i);
+	_51d_dataset_status = fiftyoneDegreesInitWithPropertyArray(global._51degrees.data_file_path, (const char**)_51d_property_list, i);
 #endif
 
 	temp = get_trash_chunk();