tarball is missing file vasprintf.h

since https://github.com/mono/mono/commit/975bf0a37966e55e5544660f3f5089a8a90f4567#diff-0, gerror.c requires vasprintf.h

But this file is currently missing in the tarball, and when building from the tarball, you get:
````
[01175] make[4]: Entering directory `/root/rpmbuild/BUILD/mono-3.6.1/eglib/src'
[01175]   CC     libeglib_la-garray.lo
[01175]   CC     libeglib_la-gbytearray.lo
[01176]   CC     libeglib_la-gerror.lo
[01176] gerror.c:33:23: error: vasprintf.h: No such file or directory
````

This pull request adds the file to the tarball.
I have tested building from the fixed tarball, and it works.