ANDROID: build: Adding mm/slab.h to unsafe headers

While it's not ideal to use the private kernel header mm/slab.h,
performance requirements in external modules require the use of the
internal slab structure.

Add mm/slab.h to the "all_headers_unsafe" list so external modules
building with "--allow_ddk_unsafe_headers" can access the header.

Change-Id: I784e3f5f895766cd13e8bd2a73a4081871c6058d
Signed-off-by: John Moon <quic_johmoo@quicinc.com>

BUILD.bazel

@@ -575,6 +575,7 @@ ddk_headers(
     hdrs = [
         "drivers/gpu/drm/virtio/virtgpu_trace.h",
         "drivers/devfreq/governor.h",
+        "mm/slab.h",
     ],
     # The list of include directories where source files can #include headers
     # from. In other words, these are the `-I` option to the C compiler.