patch 8.0.0450: v:progpath is not reliably set

Problem:    v:progpath is not reliably set.
Solution:   Read /proc/self/exe if possible. (idea by Michal Grochmal)
            Also fixes missing #if.
diff --git a/src/version.c b/src/version.c
index 54df706..257cba1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    450,
+/**/
     449,
 /**/
     448,