[gimp] libgimpthumb: argh! Copy paste error which broke the build.

[Jehan <jehanp src gnome org>]

commit 01e958422bfc52cd60cdf2cc6cac68b007d2dbf3
Author: Jehan <jehan girinstud io>
Date:   Sun Jul 28 20:41:46 2019 +0200

    libgimpthumb: argh! Copy paste error which broke the build.

 libgimpthumb/Makefile.am | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libgimpthumb/Makefile.am b/libgimpthumb/Makefile.am
index ee1cfdf35c..74df71d41f 100644
--- a/libgimpthumb/Makefile.am
+++ b/libgimpthumb/Makefile.am
@@ -75,7 +75,7 @@ include $(top_srcdir)/libgimpthumb/Makefile.gi
 
 libgimpthumb_@GIMP_API_VERSION@_la_SOURCES = \
        gimpthumb.h             \
-       $(libgimpthumb_introspectable_headers)
+       $(libgimpthumb_introspectable)
 
 libgimpthumbinclude_HEADERS = \
        gimpthumb.h             \