Add sndfile include directories

When sndfile is not installed in /usr, a compilation failure in
plugins/decoder/libsndfile may occur due to a missing specification of
the include directory. The required directory variable is now added to
the "include_directories" statement.

Signed-off-by: aneejit1 <aneejit1@gmail.com>

plugins/decoder/libsndfile/CMakeLists.txt

@@ -18,6 +18,7 @@ include_directories(
   ${CMAKE_SOURCE_DIR}/libk3bdevice
   ${TDE_INCLUDE_DIR}
   ${TQT_INCLUDE_DIRS}
+  ${SNDFILE_INCLUDE_DIRS}
 )
 
 link_directories(