ninja: Generate Vala compile rules after all header_deps are found

Same reason as generated C sources

mesonbuild/backend/ninjabackend.py

@@ -353,9 +353,11 @@ int dummy;
             src_list.append(src)
             obj_list.append(self.generate_single_compile(target, outfile, src, True,
                                                          header_deps=header_deps))
+
         # Generate compilation targets for C sources generated from Vala
         # sources. This can be extended to other $LANG->C compilers later if
         # necessary. This needs to be separate for at least Vala
+        vala_generated_source_files = []
         for src in vala_generated_sources:
             raw_src = RawFilename(src)
             src_list.append(src)
@@ -371,10 +373,15 @@ int dummy;
                 if self.environment.is_header(src):
                     header_deps.append(raw_src)
                 else:
-                    # Passing 'vala' here signifies that we want the compile
-                    # arguments to be specialized for C code generated by
-                    # valac. For instance, no warnings should be emitted.
-                    obj_list.append(self.generate_single_compile(target, outfile, raw_src, 'vala', [], header_deps))
+                    # We gather all these and generate compile rules below
+                    # after `header_deps` (above) is fully generated
+                    vala_generated_source_files.append(raw_src)
+        for src in vala_generated_source_files:
+            # Passing 'vala' here signifies that we want the compile
+            # arguments to be specialized for C code generated by
+            # valac. For instance, no warnings should be emitted.
+            obj_list.append(self.generate_single_compile(target, outfile, src, 'vala', [], header_deps))
+
         # Generate compile targets for all the pre-existing sources for this target
         for f, src in target_sources.items():
             if not self.environment.is_header(src):