Add the missing fnmatch.h to the MSYS include directory

The implementation of fnmatch() is in lib/libiberty.a, but for some reason
upstream MSYS does not ship a matching header file. So just take it from

https://github.com/gcc-mirror/gcc/blob/master/include/fnmatch.h

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>