patch 8.1.0091: MS-Windows: Cannot interrupt gdb when program is running

Problem:    MS-Windows: Cannot interrupt gdb when program is running.
Solution:   Add debugbreak() and use it in the terminal debugger.
            Respect 'modified' in a prompt buffer.
diff --git a/src/version.c b/src/version.c
index 1cc6012..3fcdb73 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    91,
+/**/
     90,
 /**/
     89,