cmake: move zstd/cjson include handling

Move the respective include_directories() stansas to the top-level
cmakefile. While this technically adds them to targets where they are
not needed, there is no harm is having them.

This maskes the find_dependency/use_includes/use_libs more consistent
across the board and helps it stand out where it's forgotten. Fixes for
which will be coming at a later date.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/plugins/relay/CMakeLists.txt

@@ -70,12 +70,10 @@ list(APPEND LINK_LIBS ${LIBGCRYPT_LDFLAGS})
 list(APPEND LINK_LIBS ${ZLIB_LIBRARY})
 
 if(ENABLE_ZSTD)
-  include_directories(${LIBZSTD_INCLUDE_DIRS})
   list(APPEND LINK_LIBS ${LIBZSTD_LDFLAGS})
 endif()
 
 if(ENABLE_CJSON)
-  include_directories(${LIBCJSON_INCLUDE_DIRS})
   list(APPEND LINK_LIBS ${LIBCJSON_LDFLAGS})
 endif()