Emscripten: add missing include for getentropy()

[kleisauke]

This header was previously transitively included to support sleep() in mimalloc/atomic.h, which is no longer present in dev3 (see commit 911443d).

Details

In file included from src/prim/prim.c:22:
src/prim/emscripten/prim.c:199:13: error: call to undeclared function 'getentropy'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  195 |   int err = getentropy(buf, buf_len);
      |             ^
1 error generated.

As noticed in PR emscripten-core/emscripten#26696.