patch 8.1.0051: MS-Windows: missing #endif

Problem:    MS-Windows: missing #endif.
Solution:   Add the #endif.
diff --git a/src/os_win32.c b/src/os_win32.c
index f340949..bece2c4 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -5322,6 +5322,7 @@
 #  endif
 	}
     }
+# endif
 }
 
     void
diff --git a/src/version.c b/src/version.c
index 29cb801..5ff56fd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    51,
+/**/
     50,
 /**/
     49,