meson: build getopt when using msvc

v4: - Don't wrap a single file in a list to match mesa style
    - Use null_dep instead of empty list

Reviewed-by: Eric Anholt <eric@anholt.net> (v3)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/meson.build

@@ -47,6 +47,11 @@ sha1_h = custom_target(
 )
 
 subdir('gtest')
+if cc.get_id() == 'msvc'
+  subdir('getopt')
+else
+  idep_getopt = null_dep
+endif
 subdir('util')
 subdir('mapi')
 # TODO: opengl