[build] exclude URL library where unused (#5351)

This sets the URL library to be included only in POSIX builds,
removing unnecessary dependencies.

src/lib/Makefile.am

@@ -32,7 +32,10 @@ SUBDIRS                                 = \
     hdlc                                  \
     platform                              \
     spinel                                \
-    url                                   \
     $(NULL)
 
+if OPENTHREAD_PLATFORM_POSIX
+SUBDIRS                                += url
+endif
+
 include $(abs_top_nlbuild_autotools_dir)/automake/post.am

src/lib/url/CMakeLists.txt

@@ -26,7 +26,7 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-add_library(openthread-url
+add_library(openthread-url EXCLUDE_FROM_ALL
     url.cpp
 )