Add sdk_version: "minimum" to liblog_headers

There are libraries built with the NDK but bundled with the platform
that were otherwise directly including the header path.  It's better
for them to include liblog_headers instead.

Test: build
Change-Id: Icca3684ba5b2539789d30de9df840da4ade0dc67
diff --git a/liblog/Android.bp b/liblog/Android.bp
index 6051ac7..59ab250 100644
--- a/liblog/Android.bp
+++ b/liblog/Android.bp
@@ -43,6 +43,7 @@
         "//apex_available:anyapex",
     ],
     min_sdk_version: "29",
+    sdk_version: "minimum",
     native_bridge_supported: true,
     export_include_dirs: ["include"],
     system_shared_libs: [],