cross-build: Define crypt_data in unistd.h for libcrypt

This is where it's defined in the base system, so is where libcrypt
expects it to exist when being built, and will be needed when being
bootstrapped in a subsequent commit.

MFC after:	1 week

tools/build/cross-build/include/common/unistd.h

@@ -39,6 +39,11 @@
 #include_next <unistd.h>
 #include <getopt.h>
 
+struct crypt_data {
+	int	initialized;	/* For compatibility with glibc. */
+	char	__buf[256];	/* Buffer returned by crypt_r(). */
+};
+
 static inline int
 check_utility_compat(const char *utility __unused)
 {