subprocess: minor cosmetic cleanup

Don't recursively include the whole stream.h header; only include what's
minimally needed, and handle the rest with forward declarations.

osdep/subprocess-posix.c

@@ -15,8 +15,6 @@
  * with mpv.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "osdep/subprocess.h"
-
 #include <spawn.h>
 #include <poll.h>
 #include <unistd.h>
@@ -25,8 +23,11 @@
 #include <errno.h>
 #include <signal.h>
 
+#include "osdep/subprocess.h"
+
 #include "osdep/io.h"
 #include "common/common.h"
+#include "stream/stream.h"
 
 // Normally, this must be declared manually, but glibc is retarded.
 #ifndef __GLIBC__

osdep/subprocess-win.c

@@ -16,16 +16,17 @@
  */
 
 #define _WIN32_WINNT 0x0600
-#include "osdep/subprocess.h"
-
 #include <windows.h>
 #include <string.h>
 
+#include "osdep/subprocess.h"
+
 #include "osdep/io.h"
 #include "osdep/atomics.h"
 
 #include "talloc.h"
 #include "common/common.h"
+#include "stream/stream.h"
 #include "misc/bstr.h"
 
 static void write_arg(bstr *cmdline, char *arg)

osdep/subprocess.h

@@ -18,7 +18,9 @@
 #ifndef MP_SUBPROCESS_H_
 #define MP_SUBPROCESS_H_
 
-#include "stream/stream.h"
+#include <stddef.h>
+
+struct mp_cancel;
 
 typedef void (*subprocess_read_cb)(void *ctx, char *data, size_t size);