CMake build system installs install library headers

MQTTAsync.h, MQTTClient.h and MQTTClientPersistence.h are now
installed. So users can include the library. The default Makefile
installs those headers, but the CMake script didn't.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>

src/CMakeLists.txt

@@ -66,6 +66,8 @@ SET_TARGET_PROPERTIES(
 INSTALL(TARGETS paho-mqtt3c paho-mqtt3a MQTTVersion
     RUNTIME DESTINATION bin
     LIBRARY DESTINATION lib)
+INSTALL(FILES MQTTAsync.h MQTTClient.h MQTTClientPersistence.h
+    DESTINATION include)
 
 IF (PAHO_WITH_SSL)
     SET(OPENSSL_LIB_SEARCH_PATH "" CACHE PATH "Directory containing OpenSSL libraries")