Add license acknowledgement of mingw for src/support/dirname.c

ref 6d6ad0b07b86a10a13d5d7a1eab90b599988f496, this file used to
be under src/flisp until JuliaLang/julia#15031 so contrib/add_license_to_files.jl
was missing it [av skip]

LICENSE.md

@@ -30,7 +30,8 @@ for exceptions.
 Julia includes code from the following projects, which have their own licenses:
 - [LDC](https://github.com/ldc-developers/ldc/blob/master/LICENSE) (for ccall/cfunction ABI definitions) [BSD-3]. The portion of code that Julia uses from LDC is [BSD-3] licensed.
 - [LLVM](http://llvm.org/releases/3.7.0/LICENSE.TXT) (for parts of src/jitlayers.cpp and src/disasm.cpp) [BSD-3, effectively]
-- [MUSL](http://git.musl-libc.org/cgit/musl/tree/COPYRIGHT) (for getopt implementations on Windows) [MIT]
+- [MUSL](http://git.musl-libc.org/cgit/musl/tree/COPYRIGHT) (for getopt implementation on Windows) [MIT]
+- [MINGW](https://sourceforge.net/p/mingw/mingw-org-wsl/ci/legacy/tree/mingwrt/mingwex/dirname.c) (for dirname implementation on Windows) [MIT]
 - [NetBSD](http://www.netbsd.org/about/redistribution.html) (for setjmp, longjmp, and strptime implementations on Windows) [BSD-3]
 
 The Julia language links to the following external libraries, which have their