Cleanup: Pass 'FILE *' instead of 'void *' for BPY_python_backtrace

This was committed as a temporary workaround in 82150f5641
as release builds were failing (only debug builds worked).

This adds `stdio.h` to the header which is now split into a file that
contains more specialized functionality.

Also move function body inside BPY_python_backtrace,
removing PyC_StackPrint as we have PyC_StackSpit() for
similar functionality that can be called from a debugger.

source/blender/python/BPY_extern_python.h

@@ -28,12 +28,15 @@ struct bContext;
 extern "C" {
 #endif
 
+/* For 'FILE'. */
+#include <stdio.h>
+
 /* bpy_interface.c */
 void BPY_python_start(int argc, const char **argv);
 void BPY_python_end(void);
 void BPY_python_reset(struct bContext *C);
 void BPY_python_use_system_env(void);
-void BPY_python_backtrace(/* FILE */ void *file);
+void BPY_python_backtrace(FILE *file);
 
 #ifdef __cplusplus
 } /* extern "C" */