can't find basename() on OpenBSD

[omar-polo]

Godot version: built from master e773bf5

OS/device including version: OpenBSD-CURRENT

Issue description:

The compilation fails because it doesn't find basename:

platform/linuxbsd/os_linuxbsd.cpp:402:21: error: use of undeclared identifier 'basename'
        String file_name = basename(p_path.utf8().get_data());

#include <libgen.h> (to bring in basename(3)) doesn't seem to fix it, since then it complains about char* vs string. I ended up with the following, which at least compiles and seems to work, but I don't know how much sense it makes.

--- os_linuxbsd.cpp.orig        Fri May 21 15:24:54 2021
+++ os_linuxbsd.cpp     Fri May 21 20:58:52 2021
@@ -397,7 +399,7 @@
        // The trash can is successfully created, now we check that we don't exceed our file name length limit.
        // If the file name is too long trim it so we can add the identifying number and ".trashinfo".
        // Assumes that the file name length limit is 255 characters.
-       String file_name = basename(p_path.utf8().get_data());
+       String file_name = p_path.get_basename();
        if (file_name.length() > 240) {
                file_name = file_name.substr(0, file_name.length() - 15);
        }

Steps to reproduce:

godot doesn't compile as-is from the master branch, it needs some patches (in particular for some third parties stuff), but none of those should cause this issue.