autotools: allow --enable-symbol-hiding with windows

This local autotools logic was put in place in
9e24b9c7afbcb81120af4cf3f6cdee49a06d8224 (in 2012) which disabled it for
Windows unconditionally. Testing reveals that it actually works with
tested toolchains (mingw-w64 and CI ones), so let's allow this build
feature on that platform. Bringing this in sync with CMake, which already
supported this.

Reviewed-by: Jay Satiro

Closes #9586

m4/curl-confopts.m4

@@ -458,8 +458,7 @@ AC_DEFUN([CURL_CONFIGURE_SYMBOL_HIDING], [
   AC_MSG_CHECKING([whether hiding of library internal symbols will actually happen])
   CFLAG_CURL_SYMBOL_HIDING=""
   doing_symbol_hiding="no"
-  if test x"$curl_cv_native_windows" != "xyes" &&
-    test "$want_symbol_hiding" = "yes" &&
+  if test "$want_symbol_hiding" = "yes" &&
     test "$supports_symbol_hiding" = "yes"; then
     doing_symbol_hiding="yes"
     CFLAG_CURL_SYMBOL_HIDING="$symbol_hiding_CFLAGS"