meson: Do not export private headers in libzstd_dep to avoid name clash

This way libzstd_dep does not override, for instance, <xxhash.h>

build/meson/lib/meson.build

@@ -124,7 +124,7 @@ libzstd = library('zstd',
   version: zstd_libversion)
 
 libzstd_dep = declare_dependency(link_with: libzstd,
-  include_directories: libzstd_includes)
+  include_directories: join_paths(zstd_rootdir,'lib')) # Do not expose private headers
 
 # we link to both:
 # - the shared library (for public symbols)