Allow static library build of virglrenderer

Using the 'library' rule instead of 'shared_library' will allow callers
to switch between static and shared library builds using the
default_library config.

It defaults to shared, so the default behavior is the same as before.

src/meson.build

@@ -184,7 +184,7 @@ libvirgl_dep = declare_dependency(
    include_directories: libvirgl_inc
 )
 
-libvirglrenderer = shared_library(
+libvirglrenderer = library(
    'virglrenderer',
    virglrenderer_sources,
    include_directories: [inc_gallium, inc_configuration],