mjit.c: fix MinGW warning by r62221

mjit.c: In function 'worker':
mjit.c:126:64: warning: passing argument 1 of '_InterlockedExchangePointer' from incompatible pointer type [-Wincompatible-pointer-types]

define MJIT_ATOMIC_SET(var, val) InterlockedExchangePointer(&(var), val)¶

                                                            ^

mjit.c:828:17: note: in expansion of macro 'MJIT_ATOMIC_SET'
MJIT_ATOMIC_SET(node->unit->iseq->body->jit_func, func);
^
In file included from C:/msys64/mingw64/x86_64-w64-mingw32/include/winnt.h:27:0,
from C:/msys64/mingw64/x86_64-w64-mingw32/include/minwindef.h:163,
from C:/msys64/mingw64/x86_64-w64-mingw32/include/windef.h:8,
from C:/msys64/mingw64/x86_64-w64-mingw32/include/windows.h:69,
from C:/msys64/mingw64/x86_64-w64-mingw32/include/winsock2.h:23,
from ./include/ruby/win32.h:41,
from ./include/ruby/defines.h:250,
from ./include/ruby/ruby.h:29,
from ./include/ruby.h:33,
from internal.h:15,
from mjit.c:76:
C:/msys64/mingw64/x86_64-w64-mingw32/include/psdk_inc/intrin-impl.h:1420:7: note: expected 'void * volatile*' but argument is of type 'VALUE ()(struct rb_execution_context_struct *, struct rb_control_frame_struct *) {aka long long unsigned int ()(struct rb_execution_context_struct *, struct rb_control_frame_struct *)}'
void *_InterlockedExchangePointer(void *volatile *Target,void *Value) {
^~~~~~~~~~~~~~~~~~~~~~~~~~~

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62239 b2dd03c8-39d4-4d8f-98ff-823fe69b080e