meson: declare dependency

This allows using libxcvt as a subproject.
Signed-off-by: Simon Ser <contact@emersion.fr>

meson.build

@@ -23,3 +23,6 @@ pkg_mod.generate(libraries : libxcvt,
                  version : '1.0',
                  name : 'libxcvt',
                  description : 'A Library to generate VESA CVT standard timing modelines.')
+
+libxcvt_dep = declare_dependency(link_with: libxcvt,
+                                 include_directories: inc)