Add the extra include directory for salad library

It's used to introduce new data structures in the Tarantool EE.

NO_DOC=no functional changes
NO_TEST=no functional changes
NO_CHANGELOG=no functional changes

Author: mkostoevr

src/box/CMakeLists.txt

@@ -136,6 +136,7 @@ include_directories(${ZSTD_INCLUDE_DIRS})
 include_directories(${PROJECT_BINARY_DIR}/src/box/sql)
 include_directories(${PROJECT_BINARY_DIR}/src/box)
 include_directories(${EXTRA_CORE_INCLUDE_DIRS})
+include_directories(${EXTRA_SALAD_INCLUDE_DIRS})
 include_directories(${EXTRA_BOX_INCLUDE_DIRS})
 
 add_library(box_error STATIC error.cc errcode.c mp_error.cc)