patch 8.2.4038: various code not used when features are disabled

Problem:    Various code not used when features are disabled.
Solution:   Add #ifdefs. (Dominique Pellé, closes #9491)
diff --git a/src/charset.c b/src/charset.c
index d762a25..12d6359 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -1476,6 +1476,7 @@
     return p;
 }
 
+#if defined(FEAT_EVAL) || defined(PROTO)
 /*
  * skip over ' ', '\t' and '\n'.
  */
@@ -1488,6 +1489,7 @@
 	++p;
     return p;
 }
+#endif
 
 /*
  * getwhitecols: return the number of whitespace