Ignore .cache used by clangd (#1018)

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

> And another question, I got many warnings while compiling wamr by clang like:
> 
> ```
> [10/67] Building C object CMakeFiles/vmlib.dir/home/huang/Work/wasm-micro-runtime/core/iwasm/common/wasm_exec_env.c.o
> In file included from /home/huang/Work/wasm-micro-runtime/core/iwasm/common/wasm_exec_env.c:6:
> In file included from /home/huang/Work/wasm-micro-runtime/core/iwasm/common/wasm_exec_env.h:11:
> /home/huang/Work/wasm-micro-runtime/core/iwasm/common/../interpreter/wasm.h:573:53: warning: unused parameter 'type_count' [-Wunused-parameter]
> wasm_get_smallest_type_idx(WASMType **types, uint32 type_count,
>                                                     ^
> 1 warning generated.
> [15/67] Building C object CMakeFiles/vmlib.dir/home/huang/Work/wasm-micro-runtime/core/iwasm/common/wasm_application.c.o
> In file included from /home/huang/Work/wasm-micro-runtime/core/iwasm/common/wasm_application.c:8:
> In file included from /home/huang/Work/wasm-micro-runtime/core/iwasm/common/../interpreter/wasm_runtime.h:9:
> /home/huang/Work/wasm-micro-runtime/core/iwasm/interpreter/wasm.h:573:53: warning: unused parameter 'type_count' [-Wunused-parameter]
> wasm_get_smallest_type_idx(WASMType **types, uint32 type_count,
>                                                     ^
> 1 warning generated.
> ```
> 
> It can be fixed simply by -Wno-unused-parameter, but should we fix it by `UNUSED(xxx)` manually ?

It would be great to fix these warnings, but it really takes efforts.

.gitignore

@@ -1,4 +1,4 @@
-
+.cache
 .vs
 .vscode
 /.idea