unix: Use standard mphalport.h header.

This also unbreaks "make minimal".

unix/mpconfigport.h

@@ -224,8 +224,6 @@ extern const struct _mp_obj_fun_builtin_t mp_builtin_open_obj;
     mp_obj_t keyboard_interrupt_obj; \
     void *mmap_region_head; \
 
-#define MICROPY_MPHALPORT_H "unix_mphal.h"
-
 // We need to provide a declaration/definition of alloca()
 #ifdef __FreeBSD__
 #include <stdlib.h>

unix/mphalport.h

@@ -23,6 +23,7 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
+#include <unistd.h>
 
 #ifndef CHAR_CTRL_C
 #define CHAR_CTRL_C (3)