bump submodule to `87c91c07663b707e831c59ec373b5e665ff9d64a` (#2828)

llm/llama.cpp

@@ -1 +1 @@
-Subproject commit b11a93df41921846a10628a7c306d5c82a549939
+Subproject commit 87c91c07663b707e831c59ec373b5e665ff9d64a