unconditionally include sys/mman.h in os-compatibility.c

Fixes:
../shared/os-compatibility.c:273:25: error: ‘PROT_READ’ undeclared (first use in this function); did you mean ‘LOCK_READ’?
  map = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, file->fd, 0);
                         ^~~~~~~~~
                         LOCK_READ

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

shared/os-compatibility.c

@@ -34,10 +34,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <libweston/zalloc.h>
-
-#ifdef HAVE_MEMFD_CREATE
 #include <sys/mman.h>
-#endif
 
 #include "os-compatibility.h"