waf: try to fix unicode/byte string messup

Some mpv builds identify with e.g. "mpv b'0.3.3' ". The version looks
like str() was called on a Python byte string. I couldn't reproduce it
on my machine (I tried with both Python 2 and 3), so I'm not exactly
sure what's going on here, but I'm hoping this commit does fix it.

waftools/generators/headers.py

@@ -6,7 +6,7 @@ def __get_version__(ctx):
                                cwd=ctx.srcnode.abspath())
     process.wait()
     (version, err) = process.communicate()
-    return version.strip()
+    return version.strip().decode("utf-8")
 
 def __get_build_date__():
     import time