meson.build: KMS support also depends on dri2

Kernel modesettings support also depends on dri2, see
./hw/xfree86/drivers/modesetting/meson.build

So update meson.build to reflect the changes made in configure.ac by
commit 9c81b8f5

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

meson.build

@@ -485,7 +485,7 @@ if not libdrm_dep.found() and libdrm_required
     error('DRI requested, but LIBDRM not found')
 endif
 
-build_modesetting = libdrm_dep.found()
+build_modesetting = libdrm_dep.found() and dri2proto_dep.found()
 
 build_vgahw = false
 if get_option('vgahw') == 'auto'