runtime: fix windows build

R=rsc, r2
CC=golang-dev
https://golang.org/cl/2135045

src/pkg/runtime/windows/mem.c

@@ -31,5 +31,5 @@ SysUnused(void *v, uintptr n)
 void
 SysFree(void *v, uintptr n)
 {
-	return stdcall(VirtualFree, 3, v, n, MEM_RELEASE);
+	stdcall(VirtualFree, 3, v, n, MEM_RELEASE);
 }

src/pkg/runtime/windows/os.h

@@ -8,6 +8,7 @@
 void *get_proc_addr(void *library, void *name);
 
 extern void *VirtualAlloc;
+extern void *VirtualFree;
 extern void *LoadLibraryEx;
 extern void *GetProcAddress;
 extern void *GetLastError;

src/pkg/runtime/windows/thread.c

@@ -14,6 +14,7 @@ void *GetStdHandle;
 void *SetEvent;
 void *WriteFile;
 void *VirtualAlloc;
+void *VirtualFree;
 void *LoadLibraryEx;
 void *GetProcAddress;
 void *GetLastError;
@@ -63,6 +64,7 @@ osinit(void)
 	GetStdHandle = get_proc_addr("kernel32.dll", "GetStdHandle");
 	SetEvent = get_proc_addr("kernel32.dll", "SetEvent");
 	VirtualAlloc = get_proc_addr("kernel32.dll", "VirtualAlloc");
+	VirtualFree = get_proc_addr("kernel32.dll", "VirtualFree");
 	WaitForSingleObject = get_proc_addr("kernel32.dll", "WaitForSingleObject");
 	WriteFile = get_proc_addr("kernel32.dll", "WriteFile");
 	GetLastError = get_proc_addr("kernel32.dll", "GetLastError");