[gimp] meson.build: fix more xmllint validation.

[Jehan <jehanp src gnome org>]

commit 2d6bf6dec5d7d1f5d94422641ebc06c7a1af1cc2
Author: Jehan <jehan girinstud io>
Date:   Fri Jun 18 16:09:25 2021 +0200

    meson.build: fix more xmllint validation.
    
    The authors.xml validation was also not run. This is nearly the last of
    getting rid of run_target(). There is still the desktop file validation
    but it doesn't have any output argument. We'll see how we update this
    last one.
    
    The only other usages of run_target() are proper usage (creating
    'install-*' targets).

 meson.build | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/meson.build b/meson.build
index e3191fdf4e..f933f1533e 100644
--- a/meson.build
+++ b/meson.build
@@ -1551,12 +1551,16 @@ custom_target('authors.md',
 )
 
 if xmllint.found()
-  run_target('validate-authors',
+  custom_target('validate-authors',
     command: [
       xmllint,
-      '--noout',
-      '--valid', 'authors.xml',
+      '--output', '@OUTPUT@',
+      '--valid', '@INPUT@',
     ],
+    input : [ 'authors.xml', ],
+    output: [ 'validate-authors-output.xml' ],
+    build_by_default: true,
+    install: false
   )
 endif