[gimp] libgimp: forgot the disable-single-include guard in gimplegacy.h

[Michael Natterer <mitch src gnome org>]

commit a42cb8051e3f38ca92fbf1682d9adbe6a451c489
Author: Michael Natterer <mitch gimp org>
Date:   Tue Jul 30 22:39:18 2019 +0200

    libgimp: forgot the disable-single-include guard in gimplegacy.h

 libgimp/gimplegacy.h | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/libgimp/gimplegacy.h b/libgimp/gimplegacy.h
index e12176a873..b46239d951 100644
--- a/libgimp/gimplegacy.h
+++ b/libgimp/gimplegacy.h
@@ -18,6 +18,10 @@
  * <https://www.gnu.org/licenses/>.
  */
 
+#if !defined (__GIMP_H_INSIDE__) && !defined (GIMP_COMPILATION)
+#error "Only <libgimp/gimp.h> can be included directly."
+#endif
+
 #ifndef __GIMP_LEGACY_H__
 #define __GIMP_LEGACY_H__