ipc: Avoid 'unistd.h' error with MSVC

Avoid compile error from MSVC:

D:\a\bitcoin\bitcoin\src\ipc\interfaces.cpp(24,1): error C1083: Cannot open include file: 'unistd.h': No such file or directory

MinGW provides this header but MSVC does not. Header is unneeded on windows
because HandleCtrlC code that uses it is not compiled on windows.

src/ipc/interfaces.cpp

@@ -21,10 +21,13 @@
 #include <memory>
 #include <stdexcept>
 #include <string>
-#include <unistd.h>
 #include <utility>
 #include <vector>
 
+#ifndef WIN32
+#include <unistd.h>
+#endif
+
 namespace ipc {
 namespace {
 #ifndef WIN32