smb: replace CURL_WIN32 with WIN32

PR #9255 aimed to fix a Cygwin/MSYS issue (#8220). It used the
`CURL_WIN32` macro, but that one is not defined here, while compiling
curl itself. This patch changes this to `WIN32`, assuming this was the
original intent.

Regression from 1c52e8a379

Reviewed-by: Marcel Raad

Closes #9701

lib/smb.c

@@ -34,7 +34,7 @@
 #include <process.h>
 #ifdef CURL_WINDOWS_APP
 #define getpid GetCurrentProcessId
-#elif defined(CURL_WIN32)
+#elif defined(WIN32)
 #define getpid _getpid
 #endif
 #endif