[gimp] configure: fix option --enable-windows-installer.

[Jehan <jehanp src gnome org>]

commit ae36d20c417d53fda79fbf87bd705a7e2d7ab094
Author: Jehan <jehan girinstud io>
Date:   Tue Mar 29 00:18:31 2022 +0200

    configure: fix option --enable-windows-installer.
    
    The name of the base source file changed as its format is now ini before
    further transformation.

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index d121a78bf8..ac6ff66eb5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2811,7 +2811,7 @@ AC_ARG_ENABLE(windows-installer,
               enable_windows_installer=no)
 
 if test "x$enable_windows_installer" != xno; then
-  if test ! -e "$srcdir/build/windows/installer/lang/setup.isl.in"; then
+  if test ! -e "$srcdir/build/windows/installer/lang/setup.isl.ini"; then
     AC_MSG_RESULT([no (not building from git)])
     AC_MSG_ERROR([Building the Windows installer is only supported when building from git.])
   fi