gdb: include gdbsupport/buildargv.h in ser-mingw.c

Fixes:

      CXX    ser-mingw.o
    /home/simark/src/binutils-gdb/gdb/ser-mingw.c: In function ‘int pipe_windows_open(serial*, const char*)’:
    /home/simark/src/binutils-gdb/gdb/ser-mingw.c:870:3: error: ‘gdb_argv’ was not declared in this scope
      870 |   gdb_argv argv (name);
          |   ^~~~~~~~

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28802
Change-Id: I7f3e8ec5f9ca8582d587545fdf6b69901259f199