glsl: Add missing cache_destroy stub function.

glsl: Add missing cache_destroy stub function.

  CC       glsl/tests/cache_test.o
glsl/tests/cache_test.c: In function ‘test_cache_create’:
glsl/tests/cache_test.c:160:4: error: implicit declaration of function ‘cache_destroy’ [-Werror=implicit-function-declaration]
    cache_destroy(cache);
    ^

Fixes: 87ab26b2ab35 ("glsl: Add initial functions to implement an on-disk cache")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>