util: make OpenSafeFile work without fcntl (Windows)

prjxray/util.py imported fcntl at module level, so merely importing
prjxray (e.g. from fasm2frames) failed on Windows, where fcntl does
not exist. The only user is OpenSafeFile's advisory flock, whose
timeout also relies on SIGALRM — equally POSIX-only.

Guard the import and skip the locking when fcntl is unavailable:
OpenSafeFile degrades to a plain open on Windows, while POSIX
behaviour is unchanged.

prjxray/util.py

@@ -8,7 +8,12 @@
 # https://opensource.org/licenses/ISC
 #
 # SPDX-License-Identifier: ISC
-import fcntl
+try:
+    import fcntl
+except ImportError:
+    # Windows: no fcntl (and no SIGALRM); OpenSafeFile degrades to a plain
+    # open without inter-process locking.
+    fcntl = None
 import math
 import os
 import random
@@ -47,6 +52,8 @@ class OpenSafeFile:
 
     def lock_file(self):
         assert self.fd is not None
+        if fcntl is None:
+            return
         try:
             signal.signal(signal.SIGALRM, timeout_handler)
             signal.alarm(self.timeout)
@@ -58,6 +65,8 @@ class OpenSafeFile:
 
     def unlock_file(self):
         assert self.fd is not None
+        if fcntl is None:
+            return
         fcntl.flock(self.fd.fileno(), fcntl.LOCK_UN)