Make winTerminate() a WINAPI function

This macro makes it use the stdcall calling convention when compiling
for 32-bit Windows. Its absence was technically a bug, but it's a no-op
on 64-bit Windows.

src/main.cpp

@@ -64,7 +64,7 @@ void terminate(int arg) {
 }
 
 #ifdef _WIN32
-static BOOL winTerminate(DWORD arg) {
+static BOOL WINAPI winTerminate(DWORD arg) {
     terminate(0);
     return FALSE;
 }