vasnprintf.c Android bug?

[Po Lu]

With the build API version set to 21 on the NDK r26b, vasnprintf.c needs
to include:

#include <wchar.h>

or else it cannot find mbstate_t.  In addition, this warning is still
emitted by Clang:

 ANDROID_CC   vasnprintf.o
vasnprintf.c:2416:21: warning: variable 'flags' set but not used 
[-Wunused-but-set-variable]
                int flags = dp->flags;