patch 8.2.4756: build error without the +eval feature

Problem:    Build error without the +eval feature.
Solution:   Adjust #ifdefs.
diff --git a/src/misc1.c b/src/misc1.c
index 9aa8ecf..632d222 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -1899,7 +1899,6 @@
     return p;
 }
 
-#if defined(FEAT_EVAL) || defined(PROTO)
     void
 vim_unsetenv(char_u *var)
 {
@@ -1925,6 +1924,7 @@
 	didset_vimruntime = FALSE;
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
 /*
  * Set environment variable "name" and take care of side effects.
  */