configure.ac needs a change for MinGW

[Eli Zaretskii]

win32-uname.[ch] were renamed in Guile 2.0.12, but configure.ac still
references them:

  case $host in
    *-*-mingw*)
      AC_CHECK_HEADER(winsock2.h, [AC_DEFINE([HAVE_WINSOCK2_H], 1,
        [Define if you have the <winsock2.h> header file.])])
      AC_CHECK_LIB(ws2_32, main)
      AC_LIBOBJ([win32-uname])  <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

This causes a fatal error during the build with MinGW.