Enforce UTF-8 encoding when opening C files

We have a couple of Python 3.x scripts that parse C files, and since C
does not have any encoding, we need to force one ourselves, to avoid the
case when we're running the build in a non-UTF-8 locale.

https://bugzilla.gnome.org/show_bug.cgi?id=792497

demos/gtk-demo/geninclude.py

@@ -33,7 +33,7 @@ demos = []
 for demo_file in in_files:
     filename =  demo_file[demo_file.rfind('/')+1:]
     demo_name = filename.replace(".c", "")
-    with open(demo_file, 'r') as f:
+    with open(demo_file, 'r', encoding='utf-8') as f:
         title = f.readline().replace("/*", "").strip()
 
 

gdk/wayland/genprotocolfiles.py

@@ -19,7 +19,7 @@ if pc.returncode != 0:
 
 # Now read the generated file again and remove all WL_EXPORTs
 content = ""
-with open(out_file, 'r') as content_file:
+with open(out_file, 'r', encoding='utf-8') as content_file:
     content = content_file.read()
 
 content = content.replace('WL_EXPORT ', '')