meson: always define libglapi

This allows the identifier to be used even if shared-glapi isn't build,
which simplifies a bunch of things.

Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mapi/meson.build

@@ -32,6 +32,8 @@ mapi_abi_py = files('mapi_abi.py')
 subdir('glapi')
 if with_shared_glapi
   subdir('shared-glapi')
+else
+  libglapi = []
 endif
 if not with_glvnd
   if with_gles1