CMake: Include parent directory of libxml2 headers as well

Else we aren't able to find it if it isn't in any of the default locations
because of the way we are including headers

For example:
LIBXML2_INCLUDE_DIR contains /usr/local/include/libxml2
... and the way we include libxml2 is ...
... which means that if /usr/local/include isn't already in our default paths, we get an error.

src/CMakeLists.txt

@@ -18,6 +18,7 @@ include_directories(
 	"${PROJECT_BINARY_DIR}/include/"      # for api.h
 	"${PROJECT_BINARY_DIR}/src/"          # for build_config.h
 	"${LIBXML2_INCLUDE_DIR}"
+	"${LIBXML2_INCLUDE_DIR}/../"
 )
 
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)