scons: Fix dependencies of marshal_generated.[ch].

These generated source files depend not only upon gl_and_es_API.xml, but
all other XML files that are included by it.

This change updates the generation rules to depend on all gen/*.xml
files, like done for other SCons generation rules, and should fix
incremental broken SCons builds due to missing dependencies.

Trivial.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index 9398ddb..fa4efe1 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -122,7 +122,7 @@
 env.CodeGenerate(
     target = 'main/marshal_generated.c',
     script = GLAPI + 'gen/gl_marshal.py',
-    source = GLAPI + 'gen/gl_and_es_API.xml',
+    source = [GLAPI + 'gen/gl_and_es_API.xml'] + env.Glob(GLAPI + 'gen/*.xml'),
     command = python_cmd + ' $SCRIPT -f $SOURCE > $TARGET'
     )
 
@@ -130,7 +130,7 @@
 env.CodeGenerate(
     target = 'main/marshal_generated.h',
     script = GLAPI + 'gen/gl_marshal_h.py',
-    source = GLAPI + 'gen/gl_and_es_API.xml',
+    source = [GLAPI + 'gen/gl_and_es_API.xml'] + env.Glob(GLAPI + 'gen/*.xml'),
     command = python_cmd + ' $SCRIPT -f $SOURCE > $TARGET'
     )