block: Fix type conflict of the copy_file_range stub

Emscripten doesn't provide copy_file_range implementation but it declares
this function in its headers. Meson correctly detects the missing
implementation and unsets HAVE_COPY_FILE_RANGE. However, the stub defined in
file-posix.c causes a type conflict with the declaration from Emscripten
during compilation.

To fix this error, this commit updates the stub implementation in
file-posix.c to exactly match the declaration in Emscripten's headers. The
manpage also aligns with this signature.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/938d2beba15d4bd496a600ee401995fbaa385c62.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

block/file-posix.c

@@ -2080,8 +2080,11 @@ static int handle_aiocb_write_zeroes_unmap(void *opaque)
 }
 
 #ifndef HAVE_COPY_FILE_RANGE
-static off_t copy_file_range(int in_fd, off_t *in_off, int out_fd,
-                             off_t *out_off, size_t len, unsigned int flags)
+#ifndef EMSCRIPTEN
+static
+#endif
+ssize_t copy_file_range(int in_fd, off_t *in_off, int out_fd,
+                        off_t *out_off, size_t len, unsigned int flags)
 {
 #ifdef __NR_copy_file_range
     return syscall(__NR_copy_file_range, in_fd, in_off, out_fd,