Add core/strings include directory

CMakeLists.txt

@@ -18,6 +18,7 @@ set (INCLUDES
 	${CMAKE_SOURCE_DIR}/src/core/mem
 	${CMAKE_SOURCE_DIR}/src/core/compressors
 	${CMAKE_SOURCE_DIR}/src/core/streams
+	${CMAKE_SOURCE_DIR}/src/core/strings
 	${CMAKE_SOURCE_DIR}/src/os
 	${CMAKE_SOURCE_DIR}/src/os/linux
 	${CMAKE_SOURCE_DIR}/src/input