[lldb] Fix building for mingw after changes to sigtstp_handler

Some signal handlers were set up within an !_MSC_VER condition,
i.e. omitted in MSVC builds but included in mingw builds. Previously
sigtstp_handler was defined in all builds, but since
4bcadd6686 / D120320 it's only
defined non platforms other than Windows.

Change the condition to !_WIN32 for consistency between the MSVC
and mingw builds, fixing the build for mingw.

Differential Revision: https://reviews.llvm.org/D122486

lldb/tools/driver/Driver.cpp

@@ -829,7 +829,7 @@ int main(int argc, char const *argv[]) {
   SBHostOS::ThreadCreated("<lldb.driver.main-thread>");
 
   signal(SIGINT, sigint_handler);
-#if !defined(_MSC_VER)
+#if !defined(_WIN32)
   signal(SIGPIPE, SIG_IGN);
   signal(SIGWINCH, sigwinch_handler);
   signal(SIGTSTP, sigtstp_handler);