firstboot: Include <crypt.h> for declaration of crypt() if needed (#7944)

Not every target system may provide a crypt() function in its stdlibc
and may use an external or replacement library, like libxcrypt, for
providing such functions.

See https://fedoraproject.org/wiki/Changes/Replace_glibc_libcrypt_with_libxcrypt.

src/firstboot/firstboot.c

@@ -23,6 +23,19 @@
 #include <shadow.h>
 #include <unistd.h>
 
+#ifdef HAVE_CRYPT_H
+/* libxcrypt is a replacement for glibc's libcrypt, and libcrypt might be
+ * removed from glibc at some point. As part of the removal, defines for
+ * crypt(3) are dropped from unistd.h, and we must include crypt.h instead.
+ *
+ * Newer versions of glibc (v2.0+) already ship crypt.h with a definition
+ * of crypt(3) as well, so we simply include it if it is present.  MariaDB,
+ * MySQL, PostgreSQL, Perl and some other wide-spread packages do it the
+ * same way since ages without any problems.
+ */
+#  include <crypt.h>
+#endif
+
 #include "sd-id128.h"
 
 #include "alloc-util.h"