According POSIX the va_list is defined in stdarg.h
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/stdarg.h.html

On some systems (like uClibc) the detection of vasprintf will fail
unless stdarg.h is included and build will later fail due to a vasprintf
redeclaration.
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Rafaël Carré <funman@videolan.org>