Fix link warnings related to streambuf stuff

[zkat]

I'm seeing the following in the build logs:

wasm-ld: warning: function signature mismatch: _ZNSt3__215basic_streambufIcNS_11char_traitsIcEEE7seekposENS_4fposI11__mbstate_tEEj
>>> defined as () -> void in ../release/libfastly-thin.a(body.cpp.obj)
>>> defined as (i32, i32, i32, i32) -> void in ../wasm32-wasip1/release/libfastly_sys.a(ios.instantiations.cpp.o)
wasm-ld: warning: function signature mismatch: _ZNSt3__215basic_streambufIcNS_11char_traitsIcEEE7seekoffExNS_8ios_base7seekdirEj
>>> defined as () -> void in ../release/libfastly-thin.a(body.cpp.obj)
>>> defined as (i32, i32, i64, i32, i32) -> void in ../wasm32-wasip1/release/libfastly_sys.a(ios.instantiations.cpp.o)
[ 98%] Built target esi
[ 99%] Building CXX object test/CMakeFiles/kv_store.dir/kv_store.cpp.obj
[100%] Linking CXX executable kv_store
wasm-ld: warning: function signature mismatch: _ZNSt3__215basic_streambufIcNS_11char_traitsIcEEE7seekposENS_4fposI11__mbstate_tEEj
>>> defined as () -> void in ../release/libfastly-thin.a(body.cpp.obj)
>>> defined as (i32, i32, i32, i32) -> void in ../wasm32-wasip1/release/libfastly_sys.a(ios.instantiations.cpp.o)
wasm-ld: warning: function signature mismatch: _ZNSt3__215basic_streambufIcNS_11char_traitsIcEEE7seekoffExNS_8ios_base7seekdirEj
>>> defined as () -> void in ../release/libfastly-thin.a(body.cpp.obj)
>>> defined as (i32, i32, i64, i32, i32) -> void in ../wasm32-wasip1/release/libfastly_sys.a(ios.instantiations.cpp.o)