[gimp] configure: fix typo which was breaking the build.

[Jehan <jehanp src gnome org>]

commit 6124be59b7616a90e62afa3c904741135180ce6b
Author: Jehan <jehan girinstud io>
Date:   Wed Nov 13 13:24:48 2019 +0100

    configure: fix typo which was breaking the build.

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 13defe956c..ae522fe7f1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1820,7 +1820,7 @@ WARNING: libheif version 1.5.0 and 1.5.1 are known to crash when
 fi
 
 if test "x$have_libheif" = xyes; then
-  FILE_HEIF='file-heif$(EXEEXT)',
+  FILE_HEIF='file-heif$(EXEEXT)'
 fi
 
 AC_SUBST(FILE_HEIF)