Add missing dependencies to meson headerscheck target

These checks compile headers, so all the headers they might include
need to have been generated.  The makefile target does that, but the
meson implementation didn't do that.  (Arguably a bit of a bug in
commit e82fc27e095.)  Add the required dependencies so that meson
compile headerscheck works in an unbuilt tree as well (as is
documented).

Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/e52e2c29-4834-4c64-a200-eddcba38fb4d%40eisentraut.org

meson.build

@@ -4138,12 +4138,24 @@ add_test_setup('running',
 ###############################################################
 
 headerscheck = files('src/tools/pginclude/headerscheck')
+
+# These checks compile headers, so all the headers they might include
+# need to have been generated.
+headerscheck_depends = [
+  generated_headers_stamp,
+  generated_backend_headers_stamp,
+  backend_parser,
+  jsonpath_gram,
+]
+
 run_target('headerscheck',
-  command: [headerscheck, meson.project_source_root(), meson.project_build_root()]
+  command: [headerscheck, meson.project_source_root(), meson.project_build_root()],
+  depends: headerscheck_depends,
 )
 
 run_target('cpluspluscheck',
-  command: [headerscheck, '--cplusplus', meson.project_source_root(), meson.project_build_root()]
+  command: [headerscheck, '--cplusplus', meson.project_source_root(), meson.project_build_root()],
+  depends: headerscheck_depends,
 )