meson: don't error out if xmllint is not found

Only needed for glib/tests and entirely optional.

glib/tests/meson.build

@@ -170,7 +170,7 @@ executable('test-spawn-echo', 'test-spawn-echo.c',
 
 # some testing of gtester functionality
 if not meson.is_cross_build() and host_system != 'windows'
-  xmllint = find_program('xmllint')
+  xmllint = find_program('xmllint', required: false)
   if xmllint.found()
     tmpsample_xml = custom_target('tmpsample.xml',
       output : 'tmpsample.xml',