Appropriate fix for msys2 bot fail on gen-def.py (#894)

src/gen-def.py

diff --git a/src/gen-def.py b/src/gen-def.py
index 74507e4..de35eb7 100755
--- a/src/gen-def.py
+++ b/src/gen-def.py
@@ -7,7 +7,7 @@
 headers_content = []
 for h in os.environ["headers"].split (' '):
 	if h.endswith (".h"):
-		with io.open (h, encoding='ISO-8859-1') as f: headers_content.append (f.read ())
+		with io.open (h, encoding='utf-8') as f: headers_content.append (f.read ())
 
 result = """EXPORTS
 %s