Silence VLA extension warnings in fusefs tests

Building tests/sys/fs/fusefs with clang 18 results the following
warning:

tests/sys/fs/fusefs/cache.cc:145:14: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
  145 |         uint8_t buf[bufsize];
      |                     ^~~~~~~

Because we do not particularly care that this is a clang extension,
suppress the warning.

MFC after: 3 days

(cherry picked from commit dc0b4094abf6784bf1a9492c2fea3fb91116b014)