Include src/ directory in the proper place

There's other code that includes headers starting from src/ directory,
so putting the corresponding include_directory() in src/effects/ is
inappropriate.

CMakeLists.txt

@@ -404,6 +404,7 @@ include_directories(BEFORE
     ${CMAKE_CURRENT_SOURCE_DIR}/src/tabbox
     ${CMAKE_CURRENT_SOURCE_DIR}/src/platformsupport
     ${CMAKE_CURRENT_SOURCE_DIR}/src/colors
+    ${CMAKE_CURRENT_SOURCE_DIR}/src
 )
 
 check_cxx_compiler_flag(-Wno-unused-parameter COMPILER_UNUSED_PARAMETER_SUPPORTED)

src/effects/CMakeLists.txt

@@ -1,8 +1,6 @@
 # KI18N Translation Domain for this library
 add_definitions(-DTRANSLATION_DOMAIN=\"kwin_effects\" -DEFFECT_BUILTINS)
 
-include_directories(${KWin_SOURCE_DIR}/src) # for xcbutils.h
-
 add_subdirectory(private)