[gimp] INSTALL: libheif must also be built with libx265.

[Jehan <jehanp src gnome org>]

commit 33749491afd3ecb7782ac07e78821d802aba8c3c
Author: Jehan <jehan girinstud io>
Date:   Wed Jun 6 21:10:46 2018 +0200

    INSTALL: libheif must also be built with libx265.

 INSTALL.in | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 687a6cadee..1fc1df3383 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -107,8 +107,9 @@ header files installed.
 
         https://github.com/strukturag/libheif
 
-     Make sure you build libheif with libde265 support, otherwise the
-     plug-in is mostly useless.
+     Make sure you build libheif with libde265 and libx265 support (for
+     respectively decoding and encoding), otherwise the plug-in is
+     mostly useless.
 
  14. The Python extension requires Python 2 development headers (@PYTHON2_REQUIRED_VERSION@
      or newer) to be present. You will also need PyGTK and the