build: correct minor typo in lttng help message

Currently the help message when using --with-lttng looks like this:

$ ./configure --help | grep 'with-lttng'
  --with-lttng          build with Lttng (Only available to Linux)

This commit makes the help message consistent with the error message
that is raised if --with-lttng is used on a non-Linux operating
system:

$ ./configure --help | grep 'with-lttng'
  --with-lttng          build with Lttng (Only available on Linux)

PR-URL: #16101
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

configure

@@ -350,7 +350,7 @@ parser.add_option('--with-dtrace',
 parser.add_option('--with-lttng',
     action='store_true',
     dest='with_lttng',
-    help='build with Lttng (Only available to Linux)')
+    help='build with Lttng (Only supported on Linux)')
 
 parser.add_option('--with-etw',
     action='store_true',