patch 8.0.1508: the :drop command is not always available

Problem:    The :drop command is not always available.
Solution:   Include :drop in all builds. (Yasuhiro Matsumoto, closes #2639)
diff --git a/src/version.c b/src/version.c
index 212e70a..963e1dc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1508,
+/**/
     1507,
 /**/
     1506,