Fix appending CMake module path to existing path

The code would simply append the folder with no separator, resulting in an invalid path if the variable was not empty.
Now properly append it.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

CMakeLists.txt

@@ -10,7 +10,7 @@ cmake_policy(SET CMP0042 NEW)
 project(mosquitto)
 set (VERSION 2.0.2)
 
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/")
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/")
 
 add_definitions (-DCMAKE -DVERSION=\"${VERSION}\")