[gimp] Eliminated possible compiler warning. From r130 of TinyScheme in SourceForge.

[Kevin Cozens <kcozens src gnome org>]

commit 38e4d5490b3f3dcca3fde38b2dc67290fd5bcd01
Author: Kevin Cozens <kevin ve3syb ca>
Date:   Mon Sep 28 15:52:35 2020 -0400

    Eliminated possible compiler warning. From r130 of TinyScheme in SourceForge.

 plug-ins/script-fu/tinyscheme/scheme.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/plug-ins/script-fu/tinyscheme/scheme.c b/plug-ins/script-fu/tinyscheme/scheme.c
index b0a5abe3fe..ed81d945c8 100644
--- a/plug-ins/script-fu/tinyscheme/scheme.c
+++ b/plug-ins/script-fu/tinyscheme/scheme.c
@@ -1943,7 +1943,9 @@ static INLINE int is_one_of(char *s, gunichar c) {
 /* skip white characters */
 static INLINE int skipspace(scheme *sc) {
      gunichar c;
+#if SHOW_ERROR_LINE
      int curr_line = 0;
+#endif
      do {
          c=inchar(sc);
 #if SHOW_ERROR_LINE